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ELECTRICITY 

THE SCIENCE OF THE NINETEENTH CENTURY 



ELECTRICITY 

THE SCIENCE 
OF THE NINETEENTH CENTURY 



A SKETCH FOR GENERAL READERS 



/V 



E. M; CAILLAKD 

AUTHOR OF 'THE INVISIBLE POWERS OF NATURE 



WITH ILLTJSTKATIONS 



NEW YORK 

D. APPLETON AND COMPANY 

1891 



'3 



Authorized Edition, 

486555 

$k 4f "SB 



PEEFACE 

The aim which the writer has proposed to herself 
in the present little work, is to give such an outline 
of modern electrical science, as may be readily under- 
stood by readers who have no previous acquaintance 
with the subject, and who, though unable to make a 
serious study of it, wish to acquire sufficient know- 
ledge to enable them to follow with intelligent 
interest the marvellous and rapid progress which 
is being made in this ever-widening field. That a 
science so comprehensive as that of electricity, 
should be exhaustively dealt with in a sketch for 
general readers is out of the question, even though 
the task had fallen to a far more competent pen 
than that of the writer. Nevertheless, a sketch 
sometimes answers a very useful purpose, in awaken- 



VI PREFACE 

ing a keen desire for a closer and fuller acquaintance 
with the truths of which it gives an indica- 
tion. Should this be the case in the present 
instance, the writer would be abundantly rewarded 
for what has been throughout a labour of love. 
At any rate most persons will agree with her, 
that to have no knowledge whatever of the striking 
advances which are being made in all branches of 
physical science, and especially in those which fall 
within the scope of " Electricity," is a considerable 
intellectual loss. It is even more than this, for 
there is no aid to faith in the Invisible greater than 
the pursuit of knowledge, which is for ever obliged 
to penetrate beyond the apparent in order to keep 
in touch with the real. 

In conclusion, the writer desires to express the 
deep obligation under which she lies to Professor 
Ayrton, for most valuable assistance in the revision 
of the proofs, without which she feels that her 
work would have far less right to be regarded with 
confidence, than she trusts is now the case. She 
has also to acknowledge the courtesy of Professor 



PREFACE Vli 

Silvamis Thompson and his publishers Messrs, Mac- 
millan, in allowing her the use of several illustra- 
tions (Figs. 11, 12, 13, and 14) from his work 
Elementary Lessons in Electricity and Magnetism ; 
and of Messrs. Siemens Brothers ; Lang, Wharton, 
and Down ; and Batley and Greenwood for the 
illustrations of dynamos, etc., in Part IV. 

EMMA MAEIE CAILLAED. 



CONTENTS 

PART I 
STATIC ELECTRICITY OR ELECTRICITY AT REST 

CHAPTER I 

PAGE 

Elementary Phenomena . 1 

CHAPTER II 

Electrical Machines and their Effects . 15 

CHAPTER III 

Electrical Charges — Sources of Electricity 

other than Friction . . .28 

CHAPTER IV 

The Leyden Jar . . . . .40 



CONTENTS 



CHAPTEE V 

PAGE 

Atmospheric Electricity . . .57 



CHAPTEE VI 

Atmospheric Electricity continued — Thunder- 
storms . . , • ,70 

CHAPTER VII 

Atmospheric Electricity continued — Dangers to 

be apprehended from lightning modes of 

Protection . . . . .91 



PART II 

MAGNETISM 

CHAPTER I 
General Properties of Magnets . .111 

CHAPTER II 

Magnetism of the Earth . . .127 



CONTENTS xi 



PART III 
CURRENT ELECTRICITY 



CHAPTER I 

PAGE 

The Galvanic Battery . . , . 138 



CHAPTER II 

Chemical and Physiological Effects of the 

Current . . , . .155 

CHAPTER III 

Magnetic Effects of the Current . .165 

CHAPTER IV 

Electro-Magnets . . , . ,178 

CHAPTER V 

Actions of Currents upon Currents — Induction 

Currents . . . . .185 

CHAPTER VI 

Practical Units of Measurement for Electric 
» 
Currents . . . . .197 



xil CONTENTS 



PART IV 
PRACTICAL APPLIANCES OF ELECTRICITY 



CHAPTER I 

PAGE 

Magneto-Electric and Dynamo-Electric Machines 

and Electro-Motors . . . .206 



CHAPTER II 

Electric Lighting . . . .222 

CHAPTER III 

Transmission of Power by Electricity . .241 

CHAPTER IV 

The Electric Telegraph . . . .250 

CHAPTER V 

The Telephone . . • . . . 266 

CHAPTER VI 

Electro-Metallurgy and Miscellaneous Appli- 
ances of Electricity . . . .282 



CONTENTS xni 



CONCLUDING CHAPTER 

"What is Electricity ? " . . . .289 



Note on the Polarisation and Magnetisation 

of Light ..... 300 



PART I 
STATIC ELECTRICITY 

OK 

ELECTRICITY AT REST 
CHAPTER I 

ELEMENTARY PHENOMENA 

Definition of Static Electricity — Electrical attraction — Known to 
the ancients as a property of amber — Gilbert's discoveries — 
Electrical repulsion — Two opposite states of electrification — 
Explanation of the terms vitreous and resinous — Superseded 
by positive and negative — Franklin's theory — Idea of excess 
and defect acknowledged in modern science — Proofs of its 
correctness — Analogy between the present state of knowledge 
of positive and negative electricity and the pre-scientific 
knowledge of heat and cold — Conductors and non-conductors 
— Meaning of charge and discharge — Induction — Action of 
points. 

Under the head of Static Electricity are classed 
those phenomena which are not caused by a con- 
tinuous flow or " current " of electricity ; and though 



2 STATIC ELECTRICITY parti 

they were the earliest known, and are frequently con- 
sidered as appertaining more or less to elementary 
knowledge, they yield neither in interest nor im- 
portance to the other branches of the science. Of 
late, special attention has been devoted to them by 
various scientific experimenters, as modern research 
leads to the opinion that here, if anywhere, will be 
found the ultimate solution of some of the many 
vexed questions surrounding that ever- recurring 
inquiry, "What is electricity?" Moreover, the 
eminently practical and useful study of atmospheric 
electricity belongs chiefly to this branch of the 
science, which cannot therefore be considered as 
either barren or uninteresting. 

The first electrical phenomenon which claims our 
attention is that of attraction. Through the whole 
domain of Nature we are familiar with attraction 
under one form or another. There is the attraction 
of the members of the solar system for each other 
and for their great centre; the attraction, between the 
earth and all terrestrial objects; the attraction between 
particles (or molecules) of matter, which enables 
them to form into the lamer and smaller masses 
which we call bodies ; lastly, there is the attraction 
between the elementary chemical atoms leading them 
to combine and recombine in an endless variety of 
ways, thus producing all the different substances 



chap. I ELEMENTARY PHENOMENA 3 

with which we are acquainted. And now on the 
threshold of this latest developed science we are 
confronted with the phenomenon of attraction again, 
in a new form and under different conditions certainly, 
but an old friend nevertheless. 

It is supposed that Thales, one of the seven sages 
of Greece, was the first to discover that amber when 
rubbed, acquired the power of attracting small light 
bodies to itself. Be this as it may, the fact was 
known to his countrymen hundreds of years before 
the Christian era ; and to the Greek word " electron," 
amber, is due the name electricity. 

Two thousand years passed away, and save that 
jet was found to share the same property as amber, 
no further advance was made in electrical knowledge. 
Then in the reign of Elizabeth, so fruitful in progress 
of all descriptions, Dr. Gilbert of Colchester, whom 
the Queen had appointed her physician chiefly out of 
admiration for his acquirements in natural science, 
added several facts to the one which had so long 
held the field alone. He found that glass, rock- 
crystal, gems, sulphur, resin, and various other sub- 
stances developed on friction the same power of 
attraction as amber and jet. He called these 
electrics. Metals and such substances as appeared 
devoid of any attractive property he termed non- 
electrics. Gilbert was wrong in this classification, 
2 



4 STATIC ELECTRICITY part i 

however, for under suitable conditions, to be presently 
described, all substances behave as electrics. Gilbert 
also ascertained that moisture prevented the success 
of his experiments, and that an electrified body, if set 
on fire, passed through a flame, or made very hot, 
lost all sign of electricity. 

It is easy to reproduce Gilbert's experiments on 
electrical attraction. A rod or tube of glass held in 
the hand and rubbed with a silk cloth, will power- 
fully attract small pieces of paper, pith, or other 
light bodies, just as a magnet will attract steel 
filings or needles, with the difference, however, that 
whereas in the magnet the force of attraction seems 
to lie in the two ends, in the rubbed glass it exists 
all over the surface, so that the pieces of paper will 
adhere to it anywhere. 

Electrified objects possess a force of repulsion as 
well as of attraction, for in making the above experi- 
ment, it will be noticed that in a very short time the 
pieces of paper or pith fall off the glass, and will not 
at once be attracted to it again. If, however, while 
they are in a state of repulsion towards the glass, a 
stick of rubbed sealing-wax be approached, they im- 
mediately fly towards and adhere to that, to be again 
soon repelled, when the glass will be once more 
found able to attract them. These phenomena point 
to the conclusion that bodies can be electrified in 



chap. I ELEMENTARY PHENOMENA 5 

two ways, and that those electrified in the same way- 
repel, while those electrified in opposite ways attract 
each other. 1 A still more striking proof is afforded 
by the fact that a couple of rubbed glass rods 
suspended by silk threads repel each other, and so 
do two rubbed sticks of sealing-wax, but the glass 
attracts the sealing-wax and the sealing-wax the 
glass. 

These opposite kinds of electricity were at first 
called vitreous and resinous, because it was believed 
that " vitreous " substances always gave signs of one 
kind of electricity and " resinous " of the other. This 
is a mistake, however. Glass rubbed with silk 
becomes " vitreously " electrified, but rubbed by fur 
becomes " resinously " electrified. It is evident, 
therefore, that the kind of electricity manifested 
depends on some relationship between the rubber 
and the object rubbed ; and it is now supposed that 
the substance whose molecules are least disturbed by 
friction shows "vitreous" electricity, and the one 
where they are most disturbed "resinous." These 
terms; vitreous and resinous have, however, quite 
fallen into disuse, and are replaced by positive and 



1 It has been proved that two small electrified bodies attract or 
repel each other with a force varying inversely as the square of 
the distance between them. This law is known as Coulomb's law, 
he having been the first to discover it. 



6 STATIC ELECTRICITY parti 

negative, known in technical works by the signs -f- 
and — . The American philosopher, Franklin, was 
the first to introduce them, as he was also the first 
to formulate a theory justifying their use. He sup- 
posed that electricity was an invisible and imponder- 
able fluid, existing in a certain fixed quantity in all 
bodies in a natural state, and that the positive state 
of electrification showed an excess, and the negative 
state a defect of this fluid. Whether electricity be 
or be not a fluid, 1 it is now agreed that it is equally 
distributed in all bodies wdiich are in a natural state, 
and that the idea of excess and defect does truly 
represent the conditions that occur in positive and 
negative electrification ; w T hich is actually the state 
of excess and which of defect not being, however, a 
matter of certainty. Practically, positively electrified 
bodies are considered and treated as those in which 
there is an excess of electricity. 

One proof of the truth of this theory of excess and 
defect lies in the fact that it is impossible to produce 

1 It is certainly not a fluid in the ordinary acceptation of the 
term ; and in speaking of electricity at all as a separate entity, care 
must be taken to remember that this is done for convenience sake. 
We speak of u electricity " as we might speak of a gale or a whirl- 
wind. These have no existence apart from the air of which 
they are certain states or conditions, and in the same way 
the various electrical phenomena are caused by conditions not of 
the air, but of another medium, to which further reference will be 
made hereafter. 



chap. I ELEMENTARY PHENOMENA 7 

a manifestation of one kind of electricity without 
causing an equal quantity of the other to appear also. 
A rubbed glass rod becomes positively electrified, but 
the silk which rubs it becomes negatively electrified, 
as can be seen by using it fastened to a glass handle 
instead of holding it in the fingers. After friction it 
will be found that both the glass rod and the silk 
will attract small neutral bodies, and that when these 
are respectively repelled, those repelled by the glass 
will be attracted by the silk and vice versa. 

Many persons seem to find a difficulty in these 
terms positive and negative, asserting that it is im- 
possible to attach a definite meaning to them, as they 
convey no clear idea to the mind. There is no doubt, 
indeed, that, if scientific men really knew what posi- 
tive and negative electricity are, they would be able 
to find better names for them. In the meanwhile, as 
the idea which is wanted to be conveyed is that the 
two electricities are of opposite kinds, perhaps the 
terms positive and negative are as good as any that 
could be put forward. One is almost afraid in the 
present state of knowledge of venturing the analogy, 
but possibly the experience which has been gone 
through in the case of heat and cold may aid the 
conception of some readers. Cold is the opposite of 
heat ; we now know that it is merely a negation, the 
absence of heat; people did not always know this. 



8 STATIC ELECTRICITY part i 

They supposed that cold was a thing in itself, but 
this error did not prevent their having a very clear 
practical conception of what cold was, and of knowing 
what they must do to neutralise it — produce heat. 
In the case of electricity the converse of this ex- 
perience is taking place. We say positive and 
negative electricity are opposite, and it has been 
assumed that the positive is the thing in itself, 
and the negative, the negation, the want of this 
thing. But this is not true ; negative electricity is 
as real as positive, though no one can pronounce what 
either is. Pending further discoveries, however, we 
may have, and electricians have, quite as good 
a working conception of positive and negative 
electricity, as the generality of mankind had of heat 
and cold before science had discovered what these 
really were. Moreover, it is perfectly understood 
what must be done to neutralise one kind of elec- 
tricity — produce the other. 

It will have been observed that a silk thread was 
recommended for suspending the glass and sealing- 
wax in one of the experiments above described, and 
that in order to discover the electrical state of the 
rubber, it must not be handled in use, but fixed to a 
glass stem. The reason is that some substances are 
conductors, and some non-conductors of electricity. 
An electrified body placed in contact with the former 



chap. I ELEMENTARY PHENOMENA 9 

at once parts with its surplus electricity to them, or, 
if it be in a state of defect, receives from them the 
electricity needful to restore it to a natural state. 
Non-conductors, on the contrary, do not allow of the 
free passage of electricity to and from them in this way, 
and consequently an electrified body, in contact with 
them only, cannot return to its normal condition until 
it has been touched by a conductor. To this latter 
class belong all metals, impure water, and charcoal. 
Animal bodies, dry wood, and a few other substances 
are partial conductors. Oils, silk, porcelain, dry air, 
and all the so-called " electrics " are non-conductors 
or insulators, thus named because an electrified body 
surrounded by them is insulated, so far as its electrical 
condition can be affected by conduction, from every 
other object ; and any body, no matter how good a 
conductor it may be, will in such a position become 
an "electric." It is for this reason that Gilbert's 
division of bodies into " electrics" and " non-electrics " 
was erroneous. Had he fastened a piece of metal to 
a clean, dry glass support, and touched it with an 
electrified body, he would have found the metal acquire 
the same property of attraction as amber or rock- 
crystal, for conductors need nothing more than contact 
at one point with electrified bodies, to become electri- 
fied themselves over their whole surface. In the case of 
non-conductors, on the contrary, every part of the 



io STATIC ELECTRICITY parti 

surface must be separately touched and excited. It is 
for this reason that friction is necessary in their case, 
and it will be the more effectual, the more markedly 
different is the electricity which they develop; for 
there are stages in this respect, some substances being 
relatively to each other, much more decidedly positive 
and much more decidedly negative than others. 

A body in an electrified state is said to be 
" charged," and it is " discharged " when it returns 
to its natural condition. At the moment of dis- 
charge a crackling noise is often heard, and if in the 
dark small sparks may be seen. The rubbing of a 
cat's back with the hand will produce these, and 
also in certain dry states of the atmosphere, combing 
the hair. Conductors are instantaneously discharged 
if touched by the hand, or by any object in connec- 
tion with the earth (i.e. in electrical connection 
by conductors, or partial conductors, such as the 
floor and walls of a house, for instance) ; but in 
the case of a "highly" charged body, it is not always 
safe to use the hand as a discharger, for the passage 
of electricity through a living body, produces curious 
and strongly marked physiological effects, and in 
some instances the disturbance may be so great as to 
occasion loss of consciousness, and even of life, as 
when a person is "struck" by lightning. It is hardly 
necessary to observe, however, that to produce such 



chap. I ELEMENTARY PHENOMENA n 

phenomena as these, an apparatus very different from 
rods of glass and sealing-wax is required; and, in fact, 
for any but the most elementary experiments an 
electrical machine is needed. Before entering into 
any details on this subject, however, some description 
must be given of what is called electrical induction. 

An electrified body brought near a conductor has 
the power of causing the latter to become electrified 
also, but in the opposite way to itself. Thus, if the 
former be positively charged, the latter will become 
negatively charged, and although, if uninsulated, it 
would be incapable under ordinary circumstances of 
retaining the electrified state, in the present instance 
it will do so, as long as it remains in the neighbourhood 
of the influencing conductor. Such a charge as this 
is called an induced charge, and electricity under such 
conditions is said to be hound, because the close 
proximity of a charge of the opposite nature, prevents 
it from availing itself of the open way of escape to 
the earth, which it would otherwise immediately take. 
If both the conductors are insulated, the effect pro- 
duced is different. Suppose the inducing charge to 
be positive as before, it cannot now give rise to an 
induced charge which is wholly negative, because 
there is no means of escape for the positive electricity 
contained in the conductor which is being influenced. 
The negative electricity of the latter is therefore 



12 



STATIC ELECTRICITY 



PART I 



attracted to the end nearest the positively charged 
conductor, and the positive electricity is repelled to 
the farther end, so that the two ends are electrified 
in opposite ways, while the middle appears to be in 
its normal condition (see Fig. 1). Could the conductor 




c 



B 



J 



m 



□ 



/ \ 



/ZZS 



Fig. 1.— Diagram illustrating the charge induced in an insulated conductor B 
by the neighbourhood of a positively electrified body A 

be divided in two, an arrangement often made to 
exhibit this phenomenon, that half of it which had 
been nearest the positively electrified body would be 
found negatively charged, and that which had been 
farthest from it, positively charged. Charge by induc- 
tion differs from that by conduction, therefore, in 
the fact that the former can be caused by altering 
the distribution of electricity, while the latter 
requires an alteration of quantity. For instance, 
if an uncharged insulated conductor were brought 
near a positively charged insulated body (as in 
Fig. 1), no electricity would be put into or taken 



chap, i ELEMENTARY PHENOMENA 13 

out of the former, so that if its power of acquir- 
ing a charge depended only on an alteration of 
the quantity of electricity possessed by it, it could 
not under these circumstances be charged at all. Yet, 
as a fact, a conductor thus placed does become 
charged, one end (or side, if it be an upright bar or 
sheet of metal) positively, and the other negatively. 
If the influencing body be taken away, the conductor 
will return to its natural electrical state ; but if the 
former be left, and the latter connected to earth, the 
conductor will become, as we have seen, negatively 
charged, and if it be then insulated again, and the 
positively charged body removed, it will retain its 
negative charge, because now an alteration has taken 
place in the quantity of electricity it possesses, some 
having escaped to the earth, and therefore while 
insulated it cannot return to its natural condition. 

We have hitherto been considering the inducing 
and induced charges, as separated by a thickness of 
air great enough to form an insurmountable barrier 
to their union. Suppose the two bodies to be 
approached nearer to each other, this barrier may 
become too slight to resist the strain which is going 
on ; and, just as the pressure of water on a dam may 
burst the dam, so the accumulated electric pressure 
bursts the insulating medium, a spark and report 
take place, and the two bodies are discharged. 



14 STATIC ELECTRICITY parti 

The subject of induced charges and the phenomena 
connected with them is full of interest, and will 
be referred to at greater length in Chapter IV. 
Meanwhile, before entering on a description of 
electrical machines, another fact of great importance 
must be stated, viz. the action of points on electricity. 
Franklin was the first to discover this, and he found 
that their effect is twofold. A pointed conductor 
both collects a far greater quantity of electricity than 
one with a flat or rounded surface, and the discharge 
from it is also much more rapid and powerful. 



CHAPTEE II 

ELECTRICAL MACHINES AND THEIR EFFECTS 

General principles of electrical machines — Yon Guericke's 
machine — Cylinder and plate machines — Use of points in 
electrical machines — Experiments with electrical machines — 
Electric chimes *— Electric windmill — Luminous effects — 
Electric spark — Brush discharge — St. Elmo's fire — Electrical 
glow — Discharge through rarefied air and gases — Return shock 
— Production of ozone — Difference between frictional and 
influence machines — The electrophorus. 

An electrical machine must always consist of two 
principal parts, one for producing, and the other for 
collecting electricity ; and in frictional madtines the 
quantity of electricity brought into play depends on 
three things — the extent of surface subjected to 
friction, the amount of friction used, and the nature 
of the two substances brought into contact. These 
should always be chosen so that the one should be 
the most positive, and the other the most nega- 
tive possible, relatively to each other. The first 
frictional machine was invented by a German, 



i6 



STATIC ELECTRICITY 



PART I 



Otto von Guericke, in 1680, and consisted of a 
large sulphur ball, supplied with a wooden axle, 
and mounted on a frame. The hand was used as 
a rubber ; and with this simple contrivance Von 
Guericke succeeded in producing much more power- 
ful effects than had ever been obtained before. 

The modern frictional machine is, however, very 
far superior to this. The surface to be rubbed 
usually consists of a large glass cylinder or plate, 




Fig. 2. — Cylinder Frictional Machine. C C, glass cylinder ; R, rubber ; 
P, prime conductor ; M, metal comb. 

provided with a handle by which it can be turned, 
and the rubber of a leather cushion or cushions, coated 
with a powdered amalgam of zinc or tin. In front 
of the glass, but not touching it, is placed the " prime 
conductor," which must of course be insulated. It 
consists in the case of the a cylinder machine " (Fig. 2) 
of a thick bar of metal, either solid or hollow, placed 
on glass supports, and provided with a row of small 
metal spikes at the end nearest the glass cylinder ; 



chap, ii ELECTRICAL MACHINES— EFFECTS 17 

and in the case of the plate machine of two bars 
similarly armed, or of one bent round so that both 
its ends should be presented to the flat surface of 
the glass, or else of a large metallic ball, on which 
a smaller one is often placed. When the handle is 
turned, positive electricity appears on the glass and 
negative on the rubbers, which are generally provided 
with a metal chain, connecting them to " earth " 
through the floor and walls of the building. The 
positive charge on the glass induces a negative charge 
on the nearest end of the prime conductor, whose 
positive electricity is repelled to the farther end, 
and is in fact the charge used for the experiments 
required. The metal points discharge the negative 
electricity at the near end, in a powerful stream on 
that part of the glass plate which is opposite to them 
for the moment, and which consequently returns to 
the rubbers as the plate continues to revolve, un- 
electrified and ready to be excited again. This is an 
important part of the arrangement, because bodies 
cannot receive an unlimited amount of electricity. 
When charged up to a certain degree (which varies 
according to the shape, size, and position of the body), 
they cannot be further electrified until discharge has 
taken place ; — and the glass plate of the frictional 
machine would reach its highest effective point, and 
be incapable of further strengthening the induced 



1 8 STATIC ELECTRICITY part i 

charge on the prime conductor in a very short time, 
if it were not for the action of the points above 
described. As the machine is provided with these, 
however, the induced positive charge at the far end 
of the conductor becomes very powerful, and long 
sparks can be drawn from it by presenting another 
conductor, and experiments performed to demonstrate 
various electrical phenomena. One sometimes made 
use of to illustrate attraction and repulsion, is the 
production of " electric chimes/' first invented by 
Franklin. Three bells are hung from the prime 
conductor, the two outer ones by wires, the inner 
one by a silk thread, and having attached to it 
a metal chain connected with the ground. Two 
brass balls hung by silk strings are placed between 
the bells, which, being positively electrified through 
their connection with the prime conductor, attract 
them, and are struck by the balls. The latter becoming 
immediately charged with the same electricity are 
repelled and attracted towards the uninsulated 
central bell, against which they strike and discharge 
themselves, when the outer bells again attract 
them. Thus they go on alternately charging and 
discharging themselves, and causing thereby the 
musical " electric chimes." 

Another experiment often shown is the electric 
windmill, which illustrates the action of points. It 



chap. II ELECTRICAL MACHINES— EFFECTS 19 

consists of straws or very light metal wires placed 
crossways and supported on a pivot, with the 
pointed ends all bent at right angles in the same 
direction. The whole arrangement is then fixed 
on the prime conductor of an electrical machine, 
and becomes strongly electrified, the greatest quantity 
of electricity collecting at the points, from which 
it streams off, causing, by the repulsion of the air- 
particles which it electrifies, a current of air known 
as an electrical "whirl." The effect of this is to 
drive the windmill rapidly round in the opposite 
direction to that of the points. Such a current is 
often strong enough to blow out the flame of a 
candle, and can always be felt by placing the hand 
in its path. « 

Besides demonstrating very strikingly electric 
attraction and repulsion, the action of points, and 
various other interesting phenomena, electrical 
machines can also produce luminous effects, which are 
simply reproductions on a small scale of the grand 
and beautiful natural appearances, caused equally by 
electricity. The electric spark has already been 
mentioned, and it is simply a miniature flash of 
lightning ; the very shape of the one, with its sinuous 
and branching appearance, irresistibly recalling the 
other, even to the most cursory observation. The 
electric spark is vivid enough to be seen in broad 
3 



20 STATIC ELECTRICITY part i 

daylight, but an equally beautiful though less bril- 
liant effect is the brush discharge, which requires a 
darkened room in order to be made visible. It is 
caused by a continuous flow of electricity from some 
conducting body. To facilitate this from an elec- 
trical machine, a piece of wire, filed at one end, is 
attached to the prime conductor, or, if the latter be 
highly charged, a bullet will answer the purpose. 
A fan-like brush of light, whose pointed end rests on 
the piece of wire or bullet, is then seen, varying in 
strength according to the nature and amount of charge 
of the conductor, and being always larger and brighter 
when the latter is positively than when it is negatively 
charged. The brush discharge is usually accompanied 
by a continuous hissing noise, very different from the 
sharp crack of the spark ; but if the conductor be 
pointed the discharge takes place silently, and is 
attended by a pale blue light, called an " electrical 
glow," which becomes a small bright star if occasioned 
by negative electricity. " St. Elmo's fire," often seen 
by sailors on the masts of their ships, is an example 
of glow discharge. It is also sometimes observed on 
trees, and more frequently on spears and lanceheads, 
or on the points of bayonets. Such appearances only 
occur when the atmospheric electricity is in a very dis- 
turbed state, most frequently before and during storms. 
The usual appearance of the electric spark is, 



chap, ii ELECTRICAL MACHINES— EFFECTS 21 

as has already been stated, that of a miniature flash 
of lightning; but if it is made to pass through a 
tube in which the air has been rarefied, a great 
change takes place. The light assumes a violet tint, 
and spreads out so as to fill the whole tube, if the 
latter be not too wide, flickering in such a way 
as to suggest the idea of undulations travelling in 
the same direction as the positive electricity. 
" Geissler's tubes " are generally used for making 
experiments of this kind. They are simply thin 
glass tubes, blown into the required shapes and 
partially exhausted of air ; into each end is fused a 
piece of platinum wire, by means of which the spark 
is conducted into the tube. Very interesting and 
beautiful effects are produced by these means. It is 
found that at the positive pole there is usually a 
single small bright star of light, while the negative 
pole is surrounded by a blue or violet-tinted glow, 
separated from the pole, however, by a small dark 
space. The more the air is rarefied, the paler does 
the luminous discharge become, and if exhaustion is 
carried to a sufficiently high pitch the whole tube 
becomes dark. The darkness appears to proceed 
from the negative pole, as with every increasing 
stage of exhaustion the dark space between it and 
the glow of light grows wider. Sometimes all the 
light in the tube breaks up into successive patches 



22 STATIC ELECTRICITY parti 

or strice, as they are called, which vibrate to and fro. 
These stria3 have their origin at the positive pole, 
and commence at a certain pitch of exhaustion, in- 
creasing in number as this increases for some time, 
when if the air or gas be still further rarefied they 
grow fewer and thicker. The colour of this lumin- 
ous discharge is found to vary with the kind of gas 
through which it passes, and also with the nature of 
the metallic conductors forming the opposite poles. 
The former cause is most active when the discharge 
is weak, and the latter when it is powerful. To 
observe the colour it is best to use narrow tubes. The 
light is seen to be of a violet tint in air and oxygen, 
blue in nitrogen, red in hydrogen, and white in 
carbonic acid. 1 

The effect produced by the metal conductors on 
the colour of the luminous discharge seems to be due 
to the vaporisation of small particles, owing to the 
intense heat developed at the respective poles, and 
indeed along the whole passage of the spark. This 
heat is so great that fine wires may be made red hot 
and even fused by it, presenting an analogy to what 
sometimes occurs in the case of lightning and light- 
ning conductors. The latter, especially in former 
days when their proper construction was less well 

1 Sparks from induction coils are more frequently used in these 
experiments than those from electrical machines. See p. 192. 



chap. II ELECTRICAL MACHINES — EFFECTS 23 

understood, have not infrequently been melted by a 
violent discharge. 

Persons standing near a powerful electrical 
machine at work often experience a curious sensation, 
as though a cobweb were spread over the face, and 
when it is discharged they perhaps feel a " shock " ; 
this is the same thing as what is known as the 
" return shock " in the case of a lightning flash, and 
is caused by induction. 1 In froth cases the presence 
of a charged body (be it cloud or electrical machine) 
causes a charge of opposite sign in other bodies 
near it, and when it is discharged they also discharge 
themselves, and in the case of a living being a 
" shock " is felt. There is also invariably a peculiar 
and powerful odour in the neighbourhood of an 
electrical machine in action, due to the presence in 
large quantities of ozone, which is a modified and, 
so to speak, condensed form of oxygen, and to which 
further reference will be made in a future chapter. 

The machines of which a slight description has 
been given above, though partly owing their efficiency 
to induction, are known by the name of frictional 
machines, since it is friction which generates and 
keeps up the supply of electricity. There are, how- 
ever, other machines much more powerful, and greatly 

1 It appears, however, that other causes may be at work to pro- 
duce the return shock. See p. 79 note. 



24 



STATIC ELECTRICITY 



PART I 



used in laboratory experiments, which are wholly 
dependent on induction ; and just as the parent of the 
perfected frictional machines of modern days was 
the homely apparatus of Otto von Guericke, so the 
progenitor of the powerful " influence " machines 
is the simple and, to all students of electricity, familiar 
little instrument known as the eleetrophomts. 

As its name indicates, it is a contrivance for 
carrying electrical charges from one place to another. 

It consists of three parts, two 
metal discs or plates, one of 
which is provided with a glass 
handle, and a slab of resin or 
ebonite — usually the latter in 
modern instruments — which 
fits into the lower plate (see 
3). The ebonite is 
by friction with 




Fig. 



Fig. 3.— Electrophorus. M, lower electrified 
metal plate ; R, resin or ebonite 

disc ; d, upper metai piate at- wool or fur, its charge being 

taclied to H, insulating handle. . 

of course negative. The 
upper metal disc is then placed upon it, but does not 
actually touch more than three or four points of 
the surface, from which it is separated by a very thin 
film of air, so that it is really in the position of an 
insulated conductor in the close neighbourhood of an 
electrified body, and becomes positively charged by 
induction, the negative electricity being repelled to 



chap, ii ELECTRICAL MACHINES— EFFECTS 25 

the outer surface. The disc is then touched with the 
finger, or in some way momentarily connected to earth 
(so that the negative electricity escapes), and being 
lifted away from the ebonite by means of the insulat- 
ing handle, is found to have retained a positive charge 
powerful enough to permit a good-sized spark to be 
drawn from it, if another conductor be presented to 
its surface. As the disc was electrified by induction, 
no part of the original charge of the ebonite has been 
taken away, and the latter will be capable of 
recharging the disc an indefinite number of times, 
without requiring to be again electrified itself. 
The simplest way of understanding the action of the 
electrophorus is to bear in mind, that when the ebonite 
has received its negative charge, and the metal plate 
attached to the glass handle is placed upon it, both 
this plate and the one under the ebonite, have positive 
charges induced on the surfaces facing the ebonite. 
Therefore, if either plate be connected to earth, and 
then insulated and removed, it will retain a positive 
charge (see p. 13). The charge thus communicated to 
the electrophorus can be conveyed to any conducting 
body and given up to it by contact, and if the process 
is repeated often enough, an insulated conductor 
may thus become electrified to a high degree. In an 
ordinary electrophorus there is no way of giving rise 
to, or increasing the original charge on the ebonite 



26 



STATIC ELECTRICITY 



PART I 



except by friction. In the accumulating influence 
machines, referred to above, however, which are 




Fig. 4. — Wimshurst Influence Machine. This machine consists of two circular 
glass plates, about \ of an inch apart, and made to revolve in opposite 
directions when the machine is at work. Each of these glass plates has 
attached to the outside a number of metallic sectors arranged like the 
spokes of a wheel. These sectors perform the office of both inductors and 
carriers, the carriers on one plate acting as inductors to the carriers on 
the opposite plate. The machine is provided with two discharging rods 
R R (whose knobs must be separated, and the rods themselves connected 
to the insulated brass cylinders L L, when the machine is to be used for 
charging any body), four collecting combs (two of which, C C, are seen in 
the figure), and two curved brass rods B B (one only represented), which 
carry at their ends small wire brushes connecting the pair of carriers 
which are at the moment under the influence of the inductors. 

made on the principle of the electrophorus, the 
initial charge is produced in quite a different way 



chap. II ELECTRICAL MACHINES— EFFECTS 27 

(explained on page 33), and is increased by a system 
of action and re -action, which enables influence 
machines to produce effects, far exceeding in magni- 
tude, any to be obtained by the same amount of 
mechanical labour from frictional machines. The best 
known influence machines are the Holz and the 
Wimshurst. The latter is represented in Fig. 4, but 
its principle cannot be understood until the explan- 
ation respecting potential, and difference of potential, 
given in the ensuing chapter has been read (p. 31). 



CHAPTEE III 

ELECTRICAL CHARGES — SOURCES OF ELECTRICITY 
OTHER THAN FRICTION 

Seat of e ' charge " — Biot's experiment — Faraday's experiment with 
conical bag — Proof-plane — Distribution of electricity on the 
surface of a sphere — On other surfaces — Density — Cause of 
charge — Analogy with dammed-up water — Importance of in- 
sulating medium — Capacity of bodies — Potential — Detection 
of charges — Gold leaf electroscope — Slightness of causes pro- 
ducing charge — Measurement of charges — Torsion balance — 
Coulomb's law — Electrometers — Difference between force and 
quantity — Various sources of electricity — Electricity not made 
but caused to manifest itself by disturbance of equilibrium — 
Analogy with air. 

Hitherto nothing has been said as to the seat of the 
charge in an electrified body, though the expressions 
used may have led to the true inference that it resides 
wholly on the surface. The interior of a conductor is 
never found to be electrified when electricity is at rest 
on it. The outer surface alone is capable of " charge/' 
This has been proved in a variety of ways. One 
experiment known as Biot's 1 is to electrify an insul- 
1 It was really first performed by Cavendish. 



chap, in ELECTRICAL CHARGES 29 

ated metal ball, over which two hemispheres also 
made of metal, and provided with glass handles can 
be fitted. So long as the hemispheres do not touch 
the ball, it retains its electrified state, but the most 
momentary contact suffices to transfer the whole 
charge to the outer surface of the hemispheres, the 
ball being left without a trace of electricity. Another 
experiment, devised by Faraday, is to electrify a 
conical linen bag placed on an insulating stand, and 
provided with silk strings by which it can be turned 
inside out. When electrified the charge is ascertained 
to be on the outside ; the strings are then pulled, so 
that what was the inner becomes the outer surface, 
and the charge is again found to be on the outside, 
showing that the electricity must have passed from 
one surface to the other, in order to retain its outside 
position. 

A third way of proving the same fact is by means 
of a little instrument called the proof-plane. This 
is a very small metal disc or bead fastened to a glass 
stem, and which when placed in contact with an 
electrified body receives a small part of the charge. 
If this disc be carefully inserted in a hollow metal 
ball which has been electrified, and be made to touch 
the inner surface, no trace of electricity is communi- 
cated to it, but it becomes charged directly by 
momentary contact with the outer surface. 



30 STATIC ELECTRICITY parti 



A sphere sufficiently far removed from other 
conductors to be practically outside the range of their 
influence, is the only body over whose surface 
electricity distributes itself with perfect equality. 
Bodies of any other shape will have more electricity 
on some parts of their surface than on others, and at 
every point or edge it will collect in greater quantities 
than anywhere else. The quantity of electricity per 
unit of area (i.e. per square centimetre,) at any given 
spot on the surface of a body, is called the density of 
electricity at that spot, and wherever the greatest 
density is, there also will the greatest effort be made 
to escape, and there will the discharge, if there is 
one, take place. 

The fact that charges reside only on the surface of 
bodies, points to the conclusion that the real seat of 
the effects produced, is not after all the conductors, as 
was formerly supposed, but the insulating medium 
by which they are surrounded. If all bodies were 
perfectly conducting, there could be no possibility of 
disturbance in electrical equilibrium, because elec- 
tricity could pass freely to and from all, thus finding 
its own equality of distribution, as water finds its 
own level. A " charge " occurs because at the sur- 
face of a conductor the electricity meets with a 
medium into which it cannot pass, and it as it were 
piles itself up so as to acquire strength to break 



chap, in ELECTRICAL CHARGES 31 

through. Dammed-up water and an electric charge 
are much in the same situation. Both are unnaturally 
confined, both will escape from confinement if they 
possibly can ; the water either by overflowing or 
bursting its banks, the electricity also by overflow, 
i.e. leakage, or by discharge, i.e. by suddenly bursting 
the imprisoning medium. Faraday was the first to 
point out that the study of the insulating medium, 
was in fact of far greater importance than that of 
conductors ; and since his time the attention of elec- 
tricians has been much more turned in this direction, 
to the great benefit both of theoretical and practical 
knowledge. 

It is evident that all electric charges cannot be 
alike ; the same body may receive either a small or 
a large charge. Again, different bodies have what is 
called a different capacity for charge, i.e. some are 
capable of accumulating a larger quantity of electricity 
than others. The capacity of a body depends partly 
on its size. The larger it is, i.e. the greater extent of 
surface it possesses, the greater the quantity of 
electricity it can receive ; but capacity is also 
affected by other causes to be mentioned hereafter. 

The electrical condition of a body compared with 
that of some other body or bodies is called its 
potential, and a charge may be one of either high or 
low potential. A small body receiving a certain 



32 STATIC ELECTRICITY parti 

amount of electricity may be at a high potential; 
a much larger body receiving the same amount of 
electricity would be at a low potential; and the 
tendency of electricity is always to flow from a body 
at high potential to one at low potential, so as to 
equalise its distribution, just as the tendency of water 
is to flow from a high to a low level for the same 
reason. In fact, it is often said that potential is to 
electricity what level is to water, and as in measur- 
ing elevations the sea-level is taken as zero, so in 
measuring differences of potential, or differences in 
electrical level, the surface of the earth, which is 
nevertheless always slightly electrified, is arbitrarily 
taken as zero. Another analogy between level and 
potential may be permitted. Falling water does work 
in passing from a higher to a lower level ; electricity 
in passing from a higher to a lower potential does 
work also, and in charging a body the same kind 
of operation is performed as in pumping up water. 
Work is expended in order that the capability for 
work may be produced. 

The potential of a conductor may be varied in one 
of three ways : (1) by altering its charge (increasing the 
charge, increases, diminishing the charge, diminishes 
the potential); (2) by altering its shape without alter- 
ing its charge (because change of shape occasions a 
change of distribution of electricity) ; (3) by altering 



chap, in ELECTRICAL CHARGES 33 

its position (because the electrical condition of a con- 
ductor is always affected by that of other bodies in 
its neighbourhood, on account of the mutual inductive 
action which takes place between them). The potential 
of any given body therefore depends on its shape, size, 
and position with reference to other bodies. 

It is now possible to explain what is the cause 
of the initial charge in an accumulating influence 
machine. It arises from the very slight potential 
difference existing between parts of the machine 
called the inductors, which fulfil the same office as 
the ebonite in the case of the electrophorus. This 
potential difference is increased by the action and 
re-action between the carriers (which correspond to 
the removable metal plates of the electrophorus,) 
and the inductors. The carriers have charges in- 
duced in them by the potential difference existing 
between the inductors. They are then made to give 
up these charges to the inductors by contact, with 
the result that the potential difference between the 
latter is increased, and their consequent inductive 
action on the carriers made stronger, so that they are 
able to receive and convey more powerful charges. 
Since this process can be indefinitely repeated, a 
very small potential difference rapidly becomes a 
very large one, and the machine consequently able 
to produce extremely powerful effects. 



34 



STA TIC ELECTRICITY 



PART I 



In electrical study and practice it is often of 
great importance to determine what the exact 
potential of a body compared with some other 
body (often the earth) is. In order to fulfil this 
purpose instruments called electrometers are used. 




Fig. 5.— Gold Leaf Electroscope, showing divergence of leaves at the 
approach of a rubbed glass rod. 

The most familiar is the " gold leaf electroscope." 1 
It consists of two strips of gold leaf placed inside 
a glass jar, and suspended by a wire which passes 
through a glass tube fixed in the cork, stopping the 

1 An electroscope is intended really to detect the presence and 
indicate the kind of electricity, but the gold leaf electroscope, 
though it can do this, is primarily a measurer of potential differ- 
ences, and therefore an electrometer. 



chap, in ELECTRICAL CHARGES 35 

inoutli of the jar. This wire terminates in a knob 
or else supports a flat piece of metal. When the 
gold leaves are unelectrified they hang straight 
down, touching each other ; but the moment any 
charge is communicated to them they diverge, being 
similarly electrified (see Fig. 5). This instrument is 
so sensitive that the smallest charge imparted to it, is 
made apparent. The chips cut from a cedar pencil 
and allowed to fall on the metal plate, are seen to be 
electrified, for as they touch the plate the leaves 
diverge. A rubbed glass rod approached within two 
or three feet from the instrument produces a marked 
effect, the gold leaves being then charged by induc- 
tion. In the old form of this instrument, however 
(as depicted in Fig. 5), difficulties arose from the fact 
that if the glass shade were made as insulating as 
possible, the gold leaves would feel the influence 
of any outside neighbouring body, and therefore there 
could be no certainty with what the one under test 
was being compared. On the other hand, if the 
insulation of the glass shade was less carefully 
attended to, then the damp or dust collected on its 
outside would bring the latter to about the same 
potential as that of the earth, with which therefore 
the body under test could be approximately com- 
pared, but then the imperfect insulation rendered 
it probable that directly the body was connected to 



36 STATIC ELECTRICITY part i 

the knob of the electroscope, it would be wholly or 
partially discharged, — and therefore no longer of 
importance to test. Both these objections to the 
gold leaf electroscope have been overcome in the 
modern form of the instrument, devised by Professors 
Ayrton and Perry, in which the interior of the glass 
shade is coated with strips of tin foil, leaving only 
enough bare space to allow the gold leaves to be 
visible, and thus screening them from the influence 
of outside bodies, while the wire supporting the gold 
leaves passes through the top of the instrument, with- 
out touching it, thus greatly facilitating insulation. 

It is curious to think that while the most advanced 
scientists are still unable to pronounce with any 
certainty what electricity is, they, and indeed every 
practical electrician can measure this mysterious 
agent with the same unfailing accuracy that a trades- 
man can weigh out a pound of tea. Given any 
known combination of circumstances, and they will 
foretell precisely the behaviour of electricity under 
those circumstances. There is a regular system of 
electro-static units which need not be entered into 
here. They are based like the practical electro- 
magnetic units, of which a list and explanation will 
be given in a subsequent portion of this work, on 
the centimetre, gramme, and second as the units of 
length, mass, and time respectively. 



chap, in ELECTRICAL CHARGES 37 

As yet no source of electricity has been mentioned 
save friction, but there are many others, among the 
most important of which are magnetism, chemical 
action, heat, and the contact of dissimilar substances. 
The two first of these will require special chapters 
devoted to them, and the subject of thermo-electricity 
also falls more properly under the head of current 
electricity. 

With regard to the contact of dissimilar substances, 
Volta was the first to discover that two metals 
allowed to touch each other become feebly electrified 
in opposite ways, there being a much greater difference 
of potential 1 between some than between others. 
Thus when zinc is placed in contact with lead, it 
becomes slightly positive relatively to the lead, in 
contact with copper it is much more decidedly 
positive, and in contact with platinum the difference 
of potential is very considerable indeed. Volta 
arranged a series of metals (to which a few have 
been added since his time), in which every metal 
becomes positive, if placed in contact with one lower 
on the list. This list is given at the end of the 
chapter, as it may be found useful for reference. 

Two dissimilar liquids in contact also show a 

1 For convenience sake a positively electrified body is usually 
said to be at high potential and a negatively electrified body at low 
potential ; but this is not really accurate, for there can be a high 
negative potential as well as a high positive one. 



38 STATIC ELECTRICITY part i 

difference of potential, as do a liquid and a metal, 
and a cold and a Lot metal. 

Other sources of electricity will be treated of as 
occasion requires, but it is worth while to notice how 
entirely we are so far justified in the conclusion, that 
electricity is not made by the exciting cause, what- 
ever that may be, but only obliged to manifest itself 
by being forced into an unnatural condition. One 
evidence of this lies in the fact that neither positive 
nor negative electricity can be produced alone, an 
equal amount of the opposite kind is invariably 
forthcoming also. Another proof of the same thing 
is the possibility of charge by induction, in which the 
electrified state can be produced by mere influence, 
without any alteration in the quantity of electricity 
present. These considerations compel us to the 
belief that whatever electricity may be, it is uni- 
versally present, though we are often unconscious 
of the fact. Nor is this in reality a strange circum- 
stance. It is probable that if the air were always in 
a state of perfect calm we should never know that 
such a thing as air existed. 1 We are rendered con- 
scious of it by disturbances in its equilibrium, which 
cause the various winds. The same observation 
seems to apply to electricity. Some cause disturbs 
its equilibrium, and we then have too much of it in 
1 See Modem Views of Electricity ', by Dr. Oliver Lodge, F.R.S. 



chap, in ELECTRICAL CHARGES 39 

one place and too little in another, and the effort to 
restore equilibrium makes us conscious that electricity 
exists ; but when it is only present in its natural 
state, i.e. in one of perfect equilibrium, we do not 
know it is there. 

List of metals in which every one is electro-positive to that 
next in order — 



Sodium 


Copper 


Magnesium 


Silver 


Zinc 


Gold 


Lead 


Platinum 


Tin 


Carbon — 


Iron 





CHAPTEE IV 

THE LEYDEN JAR 

Importance of Leyden jar — Description — Electrical forces act across 
dielectrics — Capacity of conductors increased by proximity of 
opposite charge — And by earth connection — Definition of 
condenser — Discovery of Leyden jar by Cuneus — Method 
of charging and discharging — Residual charge — Important 
part played by dielectric — Real seat of charge — Bursting of 
jars — Battery of jars — Oscillatory nature of Leyden jar dis- 
charge — Further analysis of discharge — Sparking distance — 
Cause of its increase — Analogy with recoil — Cause of damping 
out and slackening of vibrations — Experiments by Dr. Oliver 
Lodge — Discharge of jar through circuit — Wheatstone's 
experiment — Velocity of discharge — -Mechanical effects — 
Lichtenburg's figures — Magnetic effects of discharge — Physio- 
logical effects — Chemical effects. 

This name is familiar to all, not excepting those 
"born in pre-scientific days," for even then there 
were occasional quasi -scientific lectures given at 
schools and at country towns, at which the Leyden 
jar, thanks to its " shock "-giving capabilities often 
played a prominent part, though it is doubtful 
whether the audience, or even in many cases the 



chap, iv THE LEYDEN JAR 41 

" lecturer " himself, understood the principle of its 
action. To electrical students of the present day 
this simple and, as some might suppose, antiquated 
apparatus is full of interest, for it involves facts as 
important and as wide-reaching as any of the more 
famous practical appliances of modern days ; and 
only so recently as March 1889 Dr. Oliver Lodge 
in a discourse delivered at the Eoyal Institution, 
exhibited by means of the Leyden jar an entirely 
novel series of experiments illustrating some of the 
latest discoveries in electrical science. 

The ordinary Leyden jar is a common glass jar, 
coated inside and outside to about four-fifths of its 
height with tin foil, and provided with a lid of dry 
well-varnished wood, through which passes a thick 
brass wire, terminating on the outside in a metal 
knob, and communicating inside with the tinfoil, 
lining the inner surface of the jar. There are 
therefore two conductors, the two tinfoil coatings 
in presence of each other, separated by an insulating 
substance, i.e. the glass jar. In Chapter I. it has been 
stated what happens when two conductors, one of 
which is electrified, are in close neighbourhood and 
divided by the air. An induced charge in the origin- 
ally unelectrified conductor is the result. If instead 
of air a sheet of glass were placed between the 
conductors, the induction would occur just the same, 



42 STATIC ELECTRICITY fart i 

for glass and all insulating substances allow the 
electric forces to act across them, for which reason 
the name dielectrics is given to them. Though, 
however all insulators are dielectrics, all are not 
equally good in this respect, nor does the best 
insulator make the best dielectric. Dry air is a 
more powerful insulator than glass, but it is not so 
good a dielectric, i.e. the inductive force does not 
act so freely across it. This is fortunate, as if air 
were as good a dielectric as glass our thunderstorms 
Avould be more violent and frequent. Substances 
which serve well as dielectrics are said to possess 
a high inductive capacity. 

It has already been stated that charges of opposite 
nature in presence of each other are bound. They 
cannot avail themselves of the road of escape to the 
earth, even if open, because of the attraction each 
feels for the other. It follows from this, and has 
been proved by experiment, that the capacity of a 
conductor is increased by having near it another 
conductor oppositely charged. The two charges act 
on each other so strongly, they are (if the expression 
may be permitted) so occupied with each other, that 
they produce hardly any effect on surrounding objects, 
and are barely influenced by them. A conductor 
thus situated appears to be at a very much lower 
potential, than it would if it were removed from the 



chap, iv THE LEYDEN JAR 43 

neighbourhood of the oppositely charged body. If 
it is desired to raise the potential of a conductor 
when in presence of one oppositely charged, to the 
same degree as when not so, a very much larger 
quantity of electricity will be required, which is 
saying in other words that the capacity of the con- 
ductor has enormously increased. 

There is another circumstance which increases 
capacity. This is if the charged conductor be in 
presence of one not only oppositely charged, but 
connected to earth, for, supposing the latter to have 
an induced negative charge, it will if connected to 
earth lose some of its positive electricity, the negative 
charge becoming in consequence much stronger, and 
attracting yet more positive electricity into the con- 
ductor whose charge is of this sign. Consequently, a 
conductor close to one oppositely charged which has 
an earth connection, is capable of accumulating very 
large quantities of electricity, and an apparatus made 
on this principle and for this object is called an 
accumulator or more often a condenser. Having said 
thus much, we may return to the Leyden jar, which 
was the earliest known and is one of the best of all 
condensers. 

Its name is derived from the place where it was 
first invented, or to speak more correctly, discovered, 
which happened by pure accident. In the year 1746 



44 STATIC ELECTRICITY parti 

Cuneus, a scientist of Leyden, wished to electrify- 
some water. With this object he placed the liquid 
in a wide-mouthed glass vessel which he held in his 
hand, allowing a metal chain from the conductor of 
an electrical machine to dip into the water. After 
some time, thinking the latter must be sufficiently 
electrified, he took hold of the chain to lift it out of 
the vessel, when to his intense surprise he experienced 
a severe shock, which so terrified him that he let fall 
the vessel, and wrote a few days afterwards to 
Eeaumur that he would not expose himself to the 
same sensation again for the crown of France. 
What had really happened in the case of Cuneus 
and his glass vessel, was that he had unwittingly 
turned it into a condenser, the water serving as 
one conductor, his own hand as the other, and the 
glass of course as the dielectric. When there- 
fore he connected the two conductors by taking 
hold of the metal chain with his other hand, a dis- 
charge immediately took place through his body, 
occasioning the shock which so alarmed him, and 
which may be felt by any person who uses his hand 
to discharge a Leyden jar. 

The ordinary method of charging is as follows . — 
The jar is taken in the hand and its knob placed 
against the prime conductor of an electrical machine 
positively charged. Through the metal knob and 






chap, iv THE LEYDENJAR 45 

wire positive electricity passes to the inner coating 
of the jar, and induces negative electricity on the 
outer coating, driving away the positive electricity 
of the latter to earth through, the hand and bodv of 
the experimenter. 1 A stronger negative charge is the 
consequence, and its increased power of attraction 
draws more positive electricity into the inner coating, 
and the former process is repeated. The charging 
may be continued till the jar is electrified to the 
highest amount of which it is capable without burst- 
ing the glass, a contingency which always has to be 
guarded against. When charged, large sparks may 
be drawn from the jar by presenting the knuckle, or 
one of the ends of a " discharging rod " to the knob, — 
a discharging rod being simply a metal rod provided 
with glass handles, and jointed in the middle to allow 
of the two ends (which are knobbed) approaching each 
other. After the spark has been drawn the jar is 
found to be discharged or rather apparently so ; 
for if it be left some little time, and the discharging 
rod be then presented to it, a small spark may 
be drawn from it, showing that the jar could 
not have been entirely discharged by the first 
large spark. This second spark, which can never 

1 An insulated Leyden jar will not charge, because the potential 
of the two coatings rises equally, unless of course the coatings are 
connected, when they can be charged as one conductor with electricity 



of the same sign. 



4 6 



STATIC ELECTRICITY 



PART I 



be obtained immediately, is due to what is called 
the " residual charge." Its return may be hastened 
by tapping the jar, which seems to show that its 
cause must lie in the molecules of glass not being 
able to return immediately to their natural condition 
after the strain put upon them, and 
its amount depends to some extent on 
the length of time the jar has been left 
charged, but also on the kind of glass 
of which it is made. In an air-con- 
denser (a condenser formed by two 
conducting surfaces separated by air,) 
there is no residual charge. This shows 
at once that a very important part is 
played by the dielectric; and a still 
more striking proof of the same thing is 
M given by the fact that the real seat of 
the opposite charges in a Leyden jar is 
fig. 6.— Leyden no t the tinfoil coatings, but the inner 

jar with remov- 
able coatings, and outer surfaces of the jar itself, as 

M M, metallic n . . 

cases; g, glass has been proved by means of a jar with 
Jar * removable coatings (Fig. 6). If after 

charging the latter are taken away, they are found not 
to be electrified ; on being replaced their charges at 
once return. This seems to explain the reason of a 
charge being always apparently on the surface of a 
conductor ; in reality its seat is on that surface of the 




chap, iv THE LEYDEN JAR 47 

dielectric which touches the conducting surface, and 
not on the latter. It is the effort of the electricity to 
enter the dielectric medium which causes the "charge/' 
viz. an accumulation of electricity unable to disperse 
itself. The Leyden jar is in truth a type of all 
" charge " and " discharge " phenomena, and in par- 
ticular its conditions, as we shall hereafter see, are 
precisely those obtaining between two thunderclouds, 
or between a thundercloud and the earth. It is this 
typical character which invests it with so great an 
interest. A third fact concerning it which has 
already been mentioned, that if too highly charged it 
bursts, i.e. a hole is pierced in the glass dividing the 
inner and outer coatings, points to the true explana- 
tion of what happens to a dielectric placed between 
two charged conductors. It is thrown into a state of 
strain, which if too great causes it to break. In the 
case of air such a rent is self-mending ; with glass of 
course it remains, and a Leyden jar thus pierced is 
rendered useless. An instructive and significant fact 
is that a vacuum acts as a dielectric, clearly showing 
that the strain can exist without the presence of 
ordinary matter. The conclusion is therefore justi- 
fied that it must primarily take place in the ether, 1 

1 The name given to an imponderable, tenuous, and highly elastic 
medium which pervades all space and interpenetrates all matter, 
and through which heat, light, and electrical energy are propagated 
by means of radiation. According to the latest scientific theories, 



48 STATIC ELECTRICITY part i 

and be by it communicated to air, glass, and other 
dielectrics. 

If a Ley den jar be made sufficiently large, it is 
evident that it might accumulate an enormous 
quantity of electricity, but as very large jars are 
found inconvenient in practice, it is more usual to 
connect them together in such a way that they can, 
if desired, be all discharged at the same moment. 
Such an arrangement is called a " battery " of jars, 
and, if the latter be of high capacity, is a very power- 
ful source of electricity. 

The spark from a Leyden jar is of infinitesimal 
duration, lasting only a small fraction of a second, 
and it was formerly supposed to be due to a single 
discharge. Such is, however, by no means always the 
case. When the spark is examined by means of a 
very rapidly rotating mirror, it is often seen to be 
serrated, proving that the discharge which causes it is 
not a solitary rush, but a number of surgings back- 
wards and forwards, that the discharge is in fact 
oscillatory ; and the rapidity of these oscillations is 
such, that some hundreds of thousands take place 
during the minute fraction of time which limits the 
duration of the spark. This fact throws light upon 

all electrical phenomena are caused primarily by strains and stresses 
in the ether. Further reference will be made to this subject in the 
concluding chapter of the present work. 



chap, iv THE LEY DEN JAR 49 

one which would otherwise be inexplicable, viz. that a 
Leyden jar is most inclined to burst, not, as would 
be naturally supposed, before, but at the moment of 
discharge, — no doubt because the glass, though able to 
bear the continued strain in one direction (which is the 
condition of things in a " charge "), gives w r ay when 
that direction is reversed and re-reversed with such 
inconceivable rapidity, its force of recovery (or elas- 
ticity) not being equal to the demand made upon it. 

The fact of the discharge of a Leyden jar being 
oscillatory does not at first sight appear to be of any 
great importance, but when we recollect that all charge 
and discharge are like those of a Leyden jar, we begin 
to understand that such a discovery as this is of the 
highest moment, and must be intimately connected 
with any true theory of the nature of electricity. 

It is not necessary for the discharge of a Leyden 
jar to take place by means only of the discharging 
rod ; there are many other ways in which it can be 
effected, some of them very interesting and instruc- 
tive ; but there is one way in which neither it nor 
any condenser arranged in the ordinary way with one 
coating connected to the earth will discharge, and 
that is by means of a continuous flow of electricity 
to the earth or to another conducting body. 1 If a 

1 It is to be observed, however, that if a discharging rod were 
applied with great suddenness to a charged condenser of consider- 



50 STATIC ELECTRICITY parti 

wire is fastened to a single charged conductor, and 
then connected to earth or to another conductor, a 
flow of electricity begins and continues till the two 
bodies are at the same potential, just as water 
contained in two vessels connected together, one 
of which is fuller than the other, will flow from 
the fuller to the more empty vessel, till the water 
in both is at the same level. But, as we know, a 
Ley den jar is not a single charged conductor, it 
consists of two conducting surfaces separated by & 
dielectric, and it has two charges, not one charge, 
which being thus in presence of each other are bound 
and will not flow away to the earth. When the dis- 
charge takes place therefore, it is on account of the 
strain to which the dielectric separating the two elec- 
tricities is subjected, breaking it down, so that either 
a hole is pierced in the glass, or a rent made in the 
air between the knob of the jar and the discharging 
rod. It is in this kind of discharge that a spark 
passes, and the distance it can overleap is called the 
sparking distance. This increases with the difference 
of potential. A much larger spark can be drawn 
from a Leyden jar when highly charged, so that the 
coatings relatively to each other are very positive and 
very negative, than when feebly charged. It is, in 

able size, much of the discharge would then take place in the form 
of a flow between the two coatings. 



chap, iv THE LEYDEN JAR 51 

fact, difference of potential which produces the spark 
at all, and consequently the greater that difference 
is, the greater the sparking power will be. The 
well-known mechanical phenomenon of recoil helps 
to explain this. The rebound of a spring which 
has been stretched to its utmost extent is very much 
greater than when it has been only slightly stretched; 
and this analogy may help us also to a clearer idea 
of the oscillatory nature of discharge. The spring 
when let go, flies beyond its natural position and then 
back again, overshooting the mark on the other side, 
so that before it settles down a series of oscillations 
takes place. A plucked violin string gives an example 
of the same thing; and it is what frequently happens 
in the case of the Leyden jar. Its discharge is then 
a series of partial discharges, caused by the electricity 
overshooting the mark and swinging back again, just 
as the spring or the violin string overshoots the mark 
and swings back again. The inner coating of the jar 
at the instant the discharge begins is positive, it then 
becomes momentarily negative, to return again to 
positive, and then back to negative, the charge 
becoming feebler with each vibration till it is entirely 
dissipated, just as the oscillations of the spring become 
smaller and smaller in range, till they cease altogether 
and it is at rest. Clearly the greater the resistance 
the spring has to encounter in making these move- 
5 



52 STATIC ELECTRICITY parti 

ments, the fewer they will be and the sooner they 
will cease ; and we may illustrate the same fact by a 
pendulum. In air it will oscillate for a considerable 
time. In treacle it will not oscillate at all, but simply 
return to its position of equilibrium with a slow sliding 
motion. The same thing is true of a Leyden jar dis- 
charge. The electricity may encounter very little 
resistance on its road, oHt may encounter a good deal. 
In the former case the oscillations will be many and 
rapid, in the latter few and slow, and it is even possible, 
as in the case of the pendulum in treacle, to put a stop 
to them altogether. The same effect as that of resist- 
ance may be produced by weighting a spring. A 
heavy violin string vibrates much more slowly than 
a light one, and something analogous to the adding 
of weight, but which cannot here be explained, may 
be done in the case of electricity. 1 Acting on 
this principle, Dr. Oliver Lodge was able to show 
some very remarkable experiments at the Eoyal 
Institution in March 1889. He brought down the 
number of vibrations in a Leyden jar discharge from 
their usual frequency of about 1,000,000 a second to 
500 a second, with the result that the sudden sharp 
crack of the spark was changed into a distinct musical 

1 It is accomplished by increasing what some electricians call 
the " self-induction," and others the iC electro - magnetic inertia 5 * 
of the circuit. See pp. 151-152. 



chap. IV THE LEYDENJAR 53 

note, and the line of light was seen by the audience 
to be quite coarsely serrated by means of a mirror 
revolving only four times a second. 1 

It has already been said that a Leyden jar can be 
discharged in various ways : for instance, instead of 
a discharging rod being used, and one end made to 
touch the outer coating, while the other is approached 
close to the knob connected with the inner coating, 
wires may be fastened to each of the coatings and 
made to pass round a considerable space, or coiled 
many times on themselves, before their respective 
free ends are brought near together. When this is 
done, however, a spark passes between the two, just 
as it does between the discharging rod and the knob 
of the jar. Such an arrangement is called a circuit, 
and several intervals may be left in it, all of which 
the spark will overleap, provided their united length 
does not exceed that which it could pass at one jump. 

Apparently the discharge takes place as instant- 
aneously when the electricity has to travel through 
many yards of wire before arriving at " sparking 
distance " as when the discharging rod is used. This 
is not really the case, however ; and by a celebrated 
experiment (which, broadly described, consisted in 

1 For a detailed report of the above experiments and the dis- 
course which they illustrated, see The Electrician for 15th March 
1889. 



54 STATIC ELECTRICITY part i 

connecting each of the coatings of a Leyden jar with 
a considerable length of wire, and arranging three 
small intervals side by side across which sparks had 
to pass) 1 Sir Charles Wheatstone determined the 
velocity of transmission through copper w 7 ire to be 
at the rate of 463,133 kilometres, or 288,000 miles a 
second. Other experimenters have, however, obtained 
different and very much slower rates, and in any case 
it must not be supposed that this experiment really 
proved anything at all about the " velocity of elec- 
tricity." It simply showed that through a conductor 
of given resistance and capacity, a certain electric 
effect took a definite though infinitesimal time to 
travel, which would not be the same through a con- 
ductor differing in either, or both of these respects. 

The heating and luminous effects of discharge 
have been already mentioned. Its mechanical effects 
are of equal importance and interest. The electric 

1 The test was as follows : The central interval was that 
which the spark had to pass after the two electricities had 
made the journey through their respective wires ; the side 
intervals they encountered almost immediately. If the trans- 
mission had been instantaneous, the sparks examined in the 
rotating mirror would have presented three parallel lines. Instead 

of this their appearance was thus : when the mirror 

turned from right to left, and thus " when it 

turned from left to right, showing that the central spark began 
after its companions, and also proving the double flow, since one 
side interval was in the wire coming from the negative, and the 
other in that from the positive coating. 



chap, iv THE LEYDEN JAR 55 

whirl is one of these. Another is the perforating of 
paper or cardboard by passing the electric spark 
through them, and a remarkable fact is that the 
edges of the paper round the hole will be found 
turned up on both sides instead of only on one, as in 
an ordinary perforation by burning. Formerly this 
was supposed to be a proof of the double current, it 
is now, however, rather considered to be a con- 
sequence of the mechanical effects of the current 
spreading equally in all directions. Lichtenburg's 
dust figures also show the mechanical power of elec- 
tricity. They are made by means of two powders 
(often vermilion and sulphur), being shaken together 
in a muslin bag and then sifted on to a cake of resin. 
The friction causes the powder particles to become 
electrified, the vermilion positively and the sulphur 
negatively, consequently the negative parts of the 
resin attract the vermilion, and the positive the 
sulphur, so that the two powders arrange themselves 
in distinct and quite different shapes. 1 

Of other effects of discharge, that on the magnetic 
needle must be mentioned. It is deflected from its 
true position when close to the place of discharge, 
and during violent thunderstorms ships' compasses 

1 Another way of making these figures is by tracing a pattern on 
the resin with the knob of a charged Ley den jar, and then sifting 
the powder over it. 



56 STATIC ELECTRICITY part i 

have often been rendered quite useless, owing to the 
influence of the lightning on them. A steel needle 
may be made into a permanent magnet by being 
placed within a wire spiral through which a discharge 
is passed. Discharge also produces considerable 
physiological disturbances, which are most strikingly 
illustrated by lightning, and will therefore again be 
referred to under the head of atmospheric electricity ; 
but powerful electrical machines and Leyden jar 
batteries can produce effects quite as marked, and 
sometimes fatal where proper precaution is neglected. 
Even a single ordinary jar may give a very unpleasant 
shock, as Cuneus and many others have proved! 
The "return shock" experienced by persons stand- 
ing at a little distance from an electrical machine 
which is being discharged, or from some object 
"struck" by lightning is due to induction. The 
effect of the charged conductor (whether a cloud or 
the prime conductor of a machine) is to induce a 
charge of opposite kind in all neighbouring objects. 
When it discharges, they follow its example, their 
charges being no longer bound, and the consequence 
to a living body is a " shock." 

Lastly, a discharge passed through chemical com- 
pounds causes their decomposition, and its effect on 
the air, as we have already seen, is the production of 
ozone. 



CHAPTEE V 

ATMOSPHERIC ELECTRICITY 

Identity of lightning and electricity — Franklin's kite experiment — 
Its repetition by Romas — Danger of these experiments — Death 
of Richman — Existence of atmospheric electricity independ- 
ently of thunderstorms — Results of modern observations — 
Daily variations — Annual variations — Electrified clouds — Signs 
of atmospheric electricity in dry cold weather — Ozone — Its im- 
portance to health — Not found in contaminated air — Same 
true of positive electricity — Ozone an active chemical agent — 
The Aurora Borealis — Frequent appearance in high latitudes 
— Description — Resemblance to discharge through rarefied 
gases — Probable origin — Effect on the magnetic needle — Im- 
probability of its being attended by sound. 

Many early observers noticed the resemblance be- 
tween the flash and crack of the electric spark, and 
those tremendous natural manifestations lightning 
and thunder. It was reserved for the great American 
philosopher, Benjamin Franklin, however, to establish 
their identity by actual experiment. Having observed 
that lightning usually strikes the mostelevated objects, 
he resolved to erect a sort of sentry-box on some high 



58 STATIC ELECTRICITY part i 

tower, from which a pointed and insulated rod could 
be raised, and thus enable him to make his observa- 
tions. Before this could be accomplished, his fertile 
brain suggested another expedient to which resort 
could be had the first time a thundercloud approached, 
and which therefore he determined to adopt. This 
was to use a common kite, suitably prepared. He 
made one of silk, fastened an iron point to it, and 
furnished it with a string, the upper part of which 
was of ordinary twine, but the lower part of silk. At 
the junction of the two he attached a key. At the 
first sign of a thunderstorm he went out into the 
country accompanied by his son, and let fly the kite. 
At first, to Franklin's intense disappointment, no 
signs of electricity were obtained, but in a short time 
he noticed the loose fibres of the string begin to 
bristle, and holding his knuckle to the key, a bright 
spark passed between the two. Eain then fell, the 
string became wet, and its conducting powers conse- 
quently much better, and in a short time numerous 
large sparks were drawn from the key, proving beyond 
a doubt that Franklin's surmises were correct, and 
lightning and the electric spark identical. This 
celebrated experiment was made at Philadelphia in 
1752. 

In 1753 it was repeated on a very grand scale by 
the Frenchman Eomas, and in presence of several 



chap, v ATMOSPHERIC ELECTRICITY 59 

spectators, Somas himself receiving a very severe 
shock, which warned him to use a discharging 
rod instead of his knuckles to draw the sparks, 
the latter being so powerful that they appeared 
like flashes of fire a foot long, and were accom- 
panied by a noise audible at 500 feet distance. At 
the commencement of this experiment no rain was 
falling ; when it began to do so a great increase 
of electricity was perceptible, and Eomas dared not 
draw sparks even by means of the discharging rod. 
Both he and the bystanders experienced the peculiar 
sensation, " as though a spider's web had been upon 
the face," which is sometimes felt near an electrical 
machine, and the explosions and flashes of fire were 
attended by the strong and characteristic smell accom- 
panying a violent lightning discharge, and the work- 
ing of electrical machines. This odour was described 
by nearly all early observers as " sulphurous," but in 
reality it more resembles that of phosphorus, and is 
now known to be due to the production of ozone. 

It is evident that experiments of this description, 
unless conducted with the very greatest care, are in 
the highest degree dangerous, and before long a fatal 
accident checked the growing ardour of the scientific 
world for making them. On the 16th of August 
1753 Professor Eichman of St. Petersburg, while 
conducting experiments during a thunderstorm on an 



60 STATIC ELECTRICITY parti 

insulated iron rod, projecting from the roof of his 
house and carried down into the room where he 
worked, was killed by a sudden flash of electric fire 
which darted from the rod with a loud report. So 
terrible a warning was not unheeded, and experiments 
on lightning became very rare ; in fact, it may be 
said that since this period hardly any have been 
undertaken, though many facts and observations 
have been collected, and the rapid growth of electrical 
science has enabled practical men to grapple, to a 
very great extent successfully, with the problem of 
protection from injuries to life and property due to 
lightning. 

Before entering upon the subject of thunderstorms, 
which are occasioned by an abnormal electrical con- 
dition of the atmosphere, it will be well to glance at 
what that condition is when no storms are in pro- 
gress. 

The discovery that electricity exists in the atmo- 
sphere quite independently of thunderstorms was an 
almost immediate consequence of Franklin's famous 
experiment with the kite. The Abbe Mazeas dis- 
covered in 1753 that signs of atmospheric electricity 
could be obtained in fine dry weather at all hours 
between sunrise and sunset ; and Franklin himself 
attached a pointed iron rod to his house, which could 
be insulated when he chose, and to which he con- 



chap. V ATMOSPHERIC ELECTRICITY 61 

nected a system of the electric chimes previously 
described. "Whenever the conductor was affected by 
the neighbourhood of some charged cloud, the chimes 
began to ring, and Franklin's attention was drawn 
to the fact that a favourable time for his observations 
had arrived. His apparatus did not enable him to 
ascertain more than that ordinary clouds were some- 
times positive and sometimes negative, more often 
the latter ; and no very great advance was made in 
the knowledge of atmospheric electricity till the 
electroscopes and electrometers of modern days were 
applied to this purpose. And even now but little 
has been positively ascertained on this subject. 
From observations made at various times in different 
places, and compared and verified by eminent men 
of science, the following facts have, however, been 
elicited. The potential of the air increases with 
increase of distance from the surface of the earth, 
which might be caused by the latter possessing a 
negative charge, but it is not known whether this is 
really the case. In fine calm weather the atmo- 
sphere is positive ; * in cloudy, rainy, and windy 
weather it is very often negative, but will sometimes 

1 The lower strata of the atmosphere are non-conducting when 
dry. It is at some distance above the surface of the earth that 
indications of positive electricity are obtained. According to some 
recent observations, earthquakes appear to occasion a negative 
electrification of the air. 



62 STATIC ELECTRICITY part i 

change rapidly from one sign to the other. Clouds are 
always electrified, and often to a very high potential. 
Usually they are positive, but there are many excep- 
tions. The electrical condition of the air is subject 
to daily and annual variations. Every twenty-four 
hours there is a decrease of electricity, beginning an 
hour or two after midnight and continuing till shortly 
before sunrise, when an increase commences and goes 
on until some hours after sunrise, when the first 
maximum is attained. The electrical condition of 
the air then remains stationary for a short time, after 
which another decrease sets in until some hours 
after noon, when a minimum is reached. Another 
short pause ensues, and then a second increase takes 
place, attaining its maximum some hours after sun- 
set. Then occurs the second decrease, reaching its 
minimum about midnight. Many local causes, how- 
ever, may modify the ordinary electrical condition of 
the air and its daily variations. Eogs are highly 
electric, and their presence always considerably raises 
the potential of the atmosphere. Snowstorms accom- 
panied by high wind are sure to produce very strong 
indications of electricity, and this fact favours the 
idea, that one important source of atmospheric elec- 
tricity may be the friction against each other and the 
earth of solid and liquid particles, brought about by 
the wind. 



chap. V ATMOSPHERIC ELECTRICITY 63 

The annual variations of atmospheric electricity 
consist in a gradual increase beginning from May 
or June, and attaining a maximum in one of the 
winter months, varying with the locality in which 
the observations are made, and a gradual decrease 
commencing from the time of maximum, and con- 
tinuing until a minimum is reached in the early 
summer. It is therefore during the winter months 
that the air is most highly electrified, a circumstance 
which may perhaps surprise those who are accus- 
tomed to think of thunderstorms as the only signs 
of atmospheric electricity, for these in our climate 
are far more common in summer than in winter. It 
is not, however, a uniformly high potential which 
occasions storms, but great differences of potential, 
and these occur in England more frequently in sum- 
mer than in winter, owing to the larger amount of 
evaporation which is going on. Vapour in consider- 
able quantities rises from the earth and sea, drawn 
up by the heat of the sun, and on reaching the 
higher and cooler layers of air, condenses. Now, 
evaporation has been proved by experiment to be 
always attended by a development of electricity ; 
and the vapour particles are therefore all in an elec- 
trified condition. When they condense into a drop, 
their united electricity spreads itself (according to 
the laws of electrical equilibrium) over the surface of 



64 STATIC ELECTRICITY part i 

the drop, which, however small it may be, is far more 
highly electrified than any one of the vapour particles 
which have gone to form it. These minute drops 
again coalesce into larger ones, with the result of a 
still further raising of potential, and a cloud which 
is formed of an incalculable number of such electri- 
fied drops attains a very high potential indeed, and 
becomes capable of exerting a strong inductive influ- 
ence on neighbouring clouds and on the earth, and 
thus of giving rise to thunderstorms. Though 
atmospheric electricity is not indicated in this way 
during winter, or at any rate but rarely, it gives 
other less obtrusive signs of its presence. It is in the 
winter months that single well- isolated objects be- 
come most easily electrified ; every one knows that 
the peculiar crackling of the hair when combed 
occurs almost invariably in dry frosty weather, and 
articles of clothing will also then frequently show 
signs of electrification. Rubbing the hand briskly 
against flannel or any woollen material, occasions a 
crackling noise accompanied in the dark by sparks ; 
and brushing woollen garments will often cause 
them to become more dusty than before, owing to 
the friction electrifying them and making them 
attract small floating particles of matter. In New 
York, where very dry and intense cold is experienced 
in winter, these electrical effects are very marked ; 



chap, v ATMOSPHERIC ELECTRICITY 65 

the dryness of the air in dwelling-houses being in- 
creased by the method of warming, and the insulation 
of different objects by the thick woollen carpets used. 
It is stated that " if one move upon such a carpet 
with a sliding or scraping motion, and then present 
the knuckle to a metallic conductor, such as the handle 
of the door, an electric spark accompanied by a 
crackling noise will be perceived. If one goes in 
this way once or twice quickly along the carpet, the 
spark may be three-quarters of an inch long, very 
brilliant and accompanied by a tolerably loud noise." * 
In order to observe these phenomena well, the carpet 
should be entirely of wool and thick. The author- 
ity above quoted gives an amusing account of a 
visit to a lady in New York in a house where the 
conditions were particularly favourable. She drew 
brilliant sparks from the gas chandelier, a visitor 
advancing to shake hands with her, would receive a 
perceptible shock, and a spark would pass between 
herself and a lady friend bending to salute her. 

The dry cold weather which is best adapted to 
the manifestation of these and kindred phenomena 
is considered particularly favourable to health ; 
most persons then experiencing a feeling of much 
greater physical briskness and activity ; and it does 
not seem improbable that this may be" partly owing 

1 The Thunderstorm, p. 289. 



66 STATIC ELECTRICITY part i 



to the more highly electrified state of the air. In 
any case, electricity must certainly be considered an 
important salubrious agent, for to it we apparently 
owe the existence of ozone, on the necessity of which 
modern sanitarians insist so strongly. As has already 
been stated, ozone is a modified and much more 
active form of oxygen. Its presence in large quan- 
tities (as during the working of an electrical machine), 
is made known by a strong and peculiar odour, but 
in thoroughly good and pure air there is always a 
small amount of ozone, and its absence is a sign 
that the atmosphere is in some way contaminated. 
It is not found in crowded rooms, in the confined 
courts of large cities, or in any place infected by the 
breath of men or animals. In the open squares of 
towns, on bridges and quays, and, in fact, wherever 
the air is easily renovated, and consequently pure, 
indications of ozone are readily obtained ; and it is 
worth while to observe that the same remarks apply 
to positive electricity, no traces of which are dis- 
coverable in close and confined quarters, or in crowded 
streets and dwellings. Besides its health-giving 
properties^ ozone is an active chemical agent : it is 
a very rapid oxidiser, and possesses strong bleaching 
powers ; and organic substances exposed to its influ- 
ence are corroded. 

The last and most remarkable electrical pheno- 



chap, v ATMOSPHERIC ELECTRICITY 67 

menon which will be mentioned in this chapter is 
the Aurora Borealis. All travellers in the far 
north are well acquainted with its changeful and 
exquisite light, so vivid as to clothe with glory the 
winter darkness of the Arctic Eegions ; but even in 
England the Aurora is often visible, though far less 
brilliant than in higher latitudes. On one occasion, 
in early winter, the writer witnessed an exceptional 
display of its beauty. The w T hole northern horizon 
was covered with a deep rosy glow from which pale 
streamers extended far up into the heavens. Even 
this, however, was but a faint representation of the 
" northern lights " as they are known to the inhabit- 
ants of the Polar Eegions, where they are described 
as often equal to the full moon in brilliancy, appear- 
ing in the most exquisite arches, continually melting 
one into the other to reappear in new and more 
beautiful forms, and varying in colour from a silvery 
whiteness to deep shades of orange and rose colour. 
The motion of the auroral streamers w^hich start 
from the arches or glow is often exceedingly quick, 
and has gained for them among the inhabitants of 
the Shetland Isles the name of " the Merry Dancers." 
They change rapidly in form, die away in one place 
to break out in another, and are generally animated 
by a strong tremulous motion from end to end. 
This tremulous motion is like the flickering of the 



68 STATIC ELECTRICITY part i 

light caused by passing an electric spark through an 
exhausted tube, and, in fact, the two phenomena 
resemble each other almost as closely as do the elec- 
tric spark and a flash of lightning. It is therefore 
natural to refer their existence to a similar origin, 
and the Aurora Borealis is considered to be due to 
electric discharges taking place in high and there- 
fore very rarefied strata of the atmosphere. The 
actual proof that its origin is electric lies, however, 
in the fact that the presence of the Aurora invari- 
ably affects the magnetic needle, making it deviate 
from its true position, and that often to a considerable 
extent, and over very large areas of the globe. The 
Aurora does not always appear in the due north, but 
frequently towards the east or west, and sometimes 
at both points simultaneously ; neither is it always 
arched, but appears occasionally in strips, or in 
" undefined luminous clouds.'' Its position in the 
heavens is thought to affect the weather ; thus 
after an eastern Aurora, dry cold is expected, whereas 
one in the west is supposed to cause storms and 
snow, and Aurora in both quarters simultaneously, 
unsettled weather. 1 

Early travellers frequently said that a peculiar 
"hissing, crackling, and rushing noise" accompanied 
very brilliant displays of the Aurora, but no such 

1 The Thunderstorm, p. 299. 



chap, v ATMOSPHERIC ELECTRICITY 69 

sound has ever been heard by any scientifically 
trained observer, or indeed by any observer at all in 
modern times; and the fact of the Aurora taking 
place in very rarefied air, renders its being accom- 
panied by sound highly improbable. The appear- 
ance of these heavenly fires, which is frequently such 
as to give the impression of their being carried along 
by an impetuous wind, irresistibly suggests the idea 
of a " rushing " sound as their fit accompaniment, 
and this may easily have given rise to the belief that 
it had been actually heard. 1 

The Aurora is seen in the South Polar Regions as 
well as in the north, and is then called the " Aurora 
Australis ; " but these southern lights, though still 
very beautiful, are not so striking and brilliant as the 
northern. 

1 M. Gaston Plante nevertheless credits the reality of this 
sound, and attributes it, as he does that which accompanies globu- 
lar lightning, to the sudden vaporisation of liquid particles by the 
passage of the electric discharge (Phenomenes electriques de V Atmo- 
sphere, p. 146). 



CHAPTEE VI 

ATMOSPHERIC ELECTRICITY CONTINUED — 
THUNDERSTORMS 

Analogy between Leyden jar and conditions giving rise to thunder- 
storms — Reason of repeated discharge in the latter — Short 
duration of lightning — Brilliancy — Colour of lightning flashes 
— Shape — Forked lightning — Cause of ramification — Possibility 
of the light of one flash producing another — Bifurcation — Sheet 
lightning — Thunder — Globular lightning — Plante's experi- 
ments — His conclusions from them — Chemical effects of light- 
ning — Heating effects — Fulgurites — Explosive effects — 
Probable cause — Examples — Length of lightning flashes — 
Altitude of thunderclouds — Magnetic effects of lightning. 

It was stated in Chapter IV that the conditions 
giving rise to thunderstorms are the same as those of 
a charged Leyden jar. We may go a little further 
than this, however, and say that possibly the inhabit- 
ants of the earth live in a huge Leyden jar, which is 
usually but slightly charged, but may at almost any 
moment become electrified to a very high degree. The 
earth's surface, which there is reason to believe may 
be always faintly negative, would form one coating of 



chap, vi ATMOSPHERIC ELECTRICITY 71 

this jar, the clouds and upper strata of the atmo- 
sphere the other, and the air between would be the 
dielectric. Now if owing to rapid evaporation and 
condensation, or to other of the various causes which 
generate and increase atmospheric electricity, clouds 
form whose potential is very high, they act induc- 
tively on the surface of the earth and all objects 
which rise from it, and these become strongly charged 
in the opposite way, the air between the clouds and 
the earth being thrown into a state of strain exactly 
similar to that of the glass in a Leyden jar. By and 
by this strain becomes so great that the air gives 
way under it, a rent is made, a dazzling flash of 
lightning is seen, a loud roll of thunder heard, and a 
beginning of discharge is made ; but a beginning 
only, for as we well know by experience, thunder- 
clouds do not discharge themselves at once like 
Leyden jars, the lightning and thunder will often 
continue for hours with but little interval between 
the flashes, and no diminution in their brilliancy. 
The reason of this lies in the way in which clouds 
are formed of innumerable drops, each one electri- 
fied and each one insulated from its neighbour. 
Because of this, a cloud is electrified throughout its 
whole mass, and not only on its surface, therefore so 
soon as one surface discharge is over, more electricity 
replaces that which has been dispersed, and a similar 



72 STATIC ELECTRICITY parti 

state of strain recurs, to be relieved by another dis- 
charge, rarely, however, following exactly the same 
path as its predecessor. Discharges take place 
between different clouds, as well as between clouds 
and the earth. In the former case there may be 
no danger to be apprehended, but in the latter very 
serious and alarming consequences often ensue, 
though the use of lightning-rods, imperfect as the 
latter may still be in some respects, has certainly 
diminished the number of accidents to life and 
property. 

The two principal characteristics of lightning are 
its short duration and its extreme brilliancy. A 
flash of lightning is thought to last about one ten- 
thousandth part of a second. This fraction of time 
is so minute that it is difficult to form an idea of it, 
but a rou^h notion of- the momentariness of light- 
ning can be obtained, by observing that an object 
moving at however great a speed, the wheels of a 
carriage, or of a train, appear absolutely stationary 
when seen by it. Even a rifle ball would look as if 
poised motionless in mid air. One experiment which 
has been made proving this extremely short duration 
of lightning, is to observe during a flash a very 
rapidly rotating disc, painted in alternate black and 
white sectors, runuing from the centre to the edge 
like the spokes of a wheel. By daylight such a disc 



chap, vi ATMOSPHERIC ELECTRICITY 73 

appears gray while revolving, as its speed is so great 
as to allow no time for the eye to distinguish be- 
tween the black and white, which are consequently 
mixed together and form gray. Seen by a lightning 
flash, however, the black and white sectors stand out 
clearly with gray ones between them, the fraction 
of space through which one sector "is able to move 
while the flash lasts, being so small as to produce 
almost the same effect as though it were stationary. 
In order to produce exactly the same effect and have 
no gray at all, it would be necessary for the illumina- 
tion to be really instantaneous, which is impossible. 

One consequence of the short duration of light- 
ning is an apparent diminution of its brilliancy. It 
has been proved that light cannot produce its full effect 
on the eye, unless it remains at least as long as one- 
tenth of a second. But lightning lasts only the ten- 
thousandth part of a second, and it follows from this 
that we see it one hundred thousand times less 
bright than it really is. When we recollect that even 
thus diminished its brilliancy is such as to cause tem- 
porary blindness if too closely watched, we may feel 
grateful that we cannot see it in its true vividness, 
for any human powers of vision would be too weak to 
bear such a sudden and overwhelming illumination. 1 

1 It should be stated, however, that there is some evidence, 
chiefly photographic, that lightning flashes may at any rate oeca- 



74 STATIC ELECTRICITY parti 

The colour of lightning varies according to the 
condition of the atmosphere. If the latter be satur- 
ated with moisture, red will probably be the pre- 
dominating hue, because the intense heat developed 
by the passage of the flash decomposes and rarefies 
the air and the watery vapour it contains. If there- 
fore the latter be abundant, rarefied hydrogen, which 
is red when an electric discharge passes through it, 
will give its hue to the lightning. If, on the con- 
trary, the air be comparatively dry, or the quantity of 
electricity in play not very large, the colour of the 
flashes will be blue or bluish-violet, which is that of 
rarefied air during the passage of an electric dis- 
charge. 1 

The shape of a flash of forked lightning varies 
according to its length, and the equality of resistance 
it encounters in its path from a cloud to the earth, or 
from one cloud to another. If the distance be short, 
and the air of a tolerably uniform density, the flash 
may be nearly or quite straight ; but if there is a 
long way to traverse, and the air is in different states 
of density at different points (as is almost invariably 

sionally be of longer duration than is usually supposed, and the 
magnetising power of lightning also points to this conclusion. 
See the discussion on lightning - rods during the meeting of the 
British Association at Bath, 1888. — Report of the British Associ- 
ation for 1888, pp. 593, 598, 601. 

1 Plante, Phenomtnes electriques de V Atmosphere, pp. 34, 35. 



CHAP. VI 



ATMOSPHERIC ELECTRICITY 



75 



the case during a thunderstorm), the flash is sure to 
pursue a very irregular and winding path, sometimes 
seeming to meander about in the air, sometimes even 




Fig. 7.— From a photograph taken in the early morning of 7th June 1889 at 
Peterborough by Mr. A. W. Nicholls. 

making loops and knots in its progress (Fig. 9), and 
in all cases frequently giving out side flashes, as 
represented in Figs. 7 and 8. 




Fig. 8. — From a photograph taken by Mr. C. A. E. Pollock at Corpus 
College, Cambridge, on the night of 6th June 1889. 




-From a photograph by Mr. A. K. Band taken at Edinburgh 
on 6th June 1889, about 9 p.m. 



CHAP, vi ATMOSPHERIC ELECTRICITY 



77 




Fig. 10.— From a photograph taken in the early morning of 7th June 1889 at 
Peterborough, by Mr. A. W. Nicholls. 

The cause of the irregular path of a lightning 
flash lies in the tendency of electricity to take 
the path of least resistance; it would lather 
run round an obstacle, if possible, than overleap 
it, and a long electric spark will often exhibit on a 
small scale exactly the same peculiarities of shape as 
a flash of lightning, and for the same reason. The 
forking, or to speak more correctly, the branching of 
the lightning (for seen in a photograph one flash 
often seems to ramify into others, like the roots or 
branches of a tree, Fig. 7,) has a different cause. It 
is thought that one lightning flash gives rise to 
others following nearly, though not quite, the same 
direction as itself; and when we remember that the 



78 STATIC ELECTRICITY part i 

cause of lightning is the breakdown of the air under 
the strain to which it is subjected, this hypothesis 
becomes exceedingly probable, for all round the path 
of the flash, and not only in it, this same strained 
condition obtains, so that the mere shock of the air 
giving way at one point, would seem to render its 
giving way at other neighbouring points, almost a 
matter of certainty. 

There is another way in which one lightning flash 
may perhaps be the cause of others, — a way which 
is particularly interesting because it serves to show 
the exceedingly intimate connection between light 
and electricity. It is a curious and remarkable fact 
that if the light of an electric spark is made to fall 
on the space between two conductors, which are 
highly charged but just not able to spark into each 
other, they will at once do so, especially if the light fall 
on the conductors themselves, — showing that under 
certain conditions the mere effect of light is able to 
produce an electric discharge. Now, since lightning 
is in reality simply a very enormous electric spark, 
it is only reasonable to suppose that it will behave 
like one, and therefore that under some circumstances 
the light of one flash will cause another flash. The 
probable reason of this phenomenon can only be 
briefly alluded to, but it is of too great interest to 
be passed over entirely. It has already been stated 



chap, vi ATMOSPHERIC ELECTRICITY 79 

both that the discharge of a Leyden jar is often 
oscillatory and that it is the type of all discharge, 
including that of lightning. It follows then that 
lightning discharge may be oscillatory. Now in a 
medium able to transmit them oscillations or wave- 
movements spread, and with light and electricity 
the ether is the medium through w 7 hich they spread, 
as air is the medium through which sound-waves 
spread. In the case of sound, it is well known that 
if two tuning-forks of the same pitch be placed near 
each other, on striking one the other will give out its 
note also ; this is because the vibration of the first, 
communicated through the air to the second, sets up 
in that a similar vibration. It would seem that an 
electric spark or a flash of lightning caused by the 
light of another spark or flash must have an ana- 
logous origin. A vibration is set up by the first 
flash which is communicated through the ether to 
the point where the state of things is such that 
another similar vibration can be set up, and a second 
flash is the result. Such a combination of conditions 
may be rare, (it is not always that we find two tuning- 
forks near at hand able to be excited the one by the 
other,) but it is certainly possible, and its occurrence, 
if it could be proved, of the very highest interest. 1 

1 Professor Rowland appears to think that the "return shock" 
(which in the case of a lightning flash often takes place at a very 



80 STATIC ELECTRICITY parti 

A single flash of lightning will sometimes divide 
when it strikes an object, and take two or even 
more different paths for the rest of its journey to 
the earth, working destruction in each, unless very 
efficient lightning conductors be provided. Some 
description of these and the injurious effects they are 
intended to guard against, will be given shortly. In 
the meanwhile mention must be made of the second 
kind of lightning flash, the sheet, and of a third, 
which is not a flash at all and is but seldom seen, viz. 
globular lightning. 

Sheet lightning presents the appearance of a broad 
flash, emanating from the edge of the cloud, or 
occasionally from the centre, when the cloud looks 
as if it opened to allow of the exit of the electric fire. 
No object on the surface of the earth is ever struck 
by it; it is in fact a discharge between cloud and 
cloud, much of the nature of a brush discharge, and 

considerable distance from the actual point of discharge,) may be 
explained on a similar principle. In a lecture given at New York 
before the American Institute of Electrical Engineers, he said, 
" If they (the oscillations) take place, we have a ready explanation 
of what is sometimes called a back stroke of lightning. That is, 
a man at the other end of the cloud a mile or more distant from 
the lightning stroke sometimes receives a shock, or a new lightning 
flash may form at that point and kill him. This may be caused, 
according to our present theory, by the arrival of waves of 'electrical 
disturbance, which might themselves cause a slight shock, or even 
overturn the equilibrium then existing, and cause a new electric 
discharge. " 



chap, vi ATMOSPHERIC ELECTRICITY 81 

not a discharge between a cloud and the earth. 
Occasionally sheet lightning is seen on a clear sky 
and unattended by thunder, and it is then often 
called summer or heat lightning. This is no doubt 
the reflection of flashes from a storm below the 
horizon, and too distant to allow the sound of the 
thunder to reach the ear of the observer. The 
thunder itself, is supposed to be due to the sudden 
and violent expansion of the air caused by the enor- 
mous heat developed in the path of the lightning. 
The rolling sound is occasioned partly by echoes from 
the different surfaces of the clouds and from strata 
of air of unequal density, and partly by the great 
length which a lightning flash sometimes attains. 
The flash is practically simultaneous along its whole 
path, but since sound takes a very much longer time 
to travel than light, moving at the rate of only about 
1100 feet a second, the noise of the thunder though 
started at the same time throughout the entire line 
of discharge, reaches the ear by degrees, the sound 
from the nearest point first and that from the points 
farther away afterwards, according to their distance. 
Globular lightning, i.e. lightning having the ap- 
pearance of an intensely brilliant luminous or fiery 
ball, was long held to be fabulous, and the accounts 
of it have certainly often been greatly and even 
ridiculously exaggerated. Nevertheless, there re- 



82 ST A TIC ELECTRICITY part i 

mains now no manner of doubt that this phenomenon 
though rare compared to that of forked or sheet 
lightning does most unquestionably occur. 1 Persons 
trained to scientific observation have recorded its 
appearance, and their accounts tally in the main 
particulars with those of other observers. Globular 
lightning may either descend from the clouds 2 or 
ascend from the earth, 3 or it may float along at a 
greater or less distance from the soil. 4 Its move- 
ment is slow and its path often extremely capricious. 
Its duration varies from a few seconds to a minute 
or more. Its disappearance is sometimes noiseless 
and harmless, sometimes attended by a loud explo- 

1 The writer in the year 1870 herself witnessed an appearance of 
globular lightning. It occurred in Wiltshire during a heavy 
thunderstorm which, not being immediately overhead, allowed of a 
continued observation of the lightning flashes. These presented 
nothing specially remarkable. The storm was in the neighbour- 
hood of the Westbury Downs, at some distance above which 
there was a thick, leaden canopy of cloud. Towards the end 
of the storm there fell from this cloud an egg-shaped luminous 
body which apparently dropped to some point on the hills, where it 
disappeared. Some seconds afterwards there ensued a long, heavy 
roll of thunder. As the hills are distant about three miles, as the 
crow flies, from the place where the writer was situated, this lumin- 
ous globe would appear to have been of considerable size. Its 
brilliancy was apparently that of ordinary forked lightning observed 
from the same distance, but its motion very much slower, the globe 
remaining visible for two or three seconds. No rain was falling 
at the time. 

2 Phenomenes glectrigues de V At7)iosphere, p. 201 ff. 

3 Ibid. p. 214 ff. Forked lightning also, though rarely, ascends. 

4 Ibid. p. 47. 






chap. VI ATMOSPHERIC ELECTRICITY 83 

sion and disastrous effects. Some accounts have 
described these fiery balls entering the window of a 
house and going up the chimney. This is probable 
enough, as the column of hot air would make an 
excellent conducting path for the electric discharge. 
Owing to the same cause, no doubt, there have been 
accounts of similar visitors coming down the chimney, 
to the great consternation of the witnesses. Globular 
lightning has been observed in the centre of thick 
clouds during a thunderstorm, and passing between 
one cloud and another, 1 and also occasionally with a 
rotatory movement. 2 

Many of the phenomena of globular lightning 
have been artificially reproduced on a small scale by 
the late eminent French electrician, M. Gaston 
Plante, whose experiments were, however, conducted 
by the aid of current, not static electricity. He 
used powerful secondary batteries 3 which give 
currents of a very high potential, and succeeded by 
their means in producing small and intensely brilliant 
globules of fire on the surface of water, on a sheet of - 
mica placed between two conducting surfaces, and 
lastly in the air-space between two damp sheets of 
filtering paper. 4 These luminous globules behaved 

1 Plienomenes electriques de V Atmosphere, p. 207. 

2 Ibid. p. 202. 

3 See Part III. chap. i. p. 148. 

4 Phenomenes electriques de V Atmosphere, chap. i. 

1 



84 STATIC ELECTRICITY part i 

in the same manner as globular lightning. They 
moved slowly, followed, in the second and third 
cases, very irregular paths, and their duration was 
considerable. M. PI ante was led by his experiments 
to the conclusion that globular lightning must be 
produced by " dynamical " electricity {i.e. current elec- 
tricity) of very high potential and in large quantities, 
and that the fiery globes themselves are formed of 
" rarefied incandescent air, and of the gases resulting 
from the decomposition of vapour of water also in a 
state of rarefaction and incandescence." 1 The bril- 
liancy of these globes is due, according to M. Plants, 
to the quantity of electricity in play at the time of 
their appearance, and it is a fact that they are only 
observed during storms of exceptional severity. The 
rustling noise which often accompanies them he 
refers to the rapid vaporisation of liquid and solid 
particles in the path of the electric discharge, and 
the variation of colour to the same cause which pro- 
duces it in ordinary lightning. 2 

Lightning produces chemical changes similar to, 
but on an enormously larger scale than those of an 
electric spark. It is often attended by a copious 
generation of ozone, to which fact may be referred 
the powerful odour often mentioned by persons who 

1 Phenomenes eledriques dc V Atmosphere, pp. 29, 30. 
2 See p. 74. 



chap, vi ATMOSPHERIC ELECTRICITY 85 

have been near an object " struck " by lightning. 
It also decomposes the oxygen and nitrogen of the 
air in order to form nitric acid, strong traces of which 
are found in specimens of rain-water collected during 
thunderstorms, while at other times it is either 
entirely absent, or present in almost infinitesimal 
quantities. The heating effects of lightning are also 
very great ; it fuses and even vaporises metals ; but 
perhaps the most wonderful examples of its power in 
this way are the fulgurites or tubes of vitrified sand 
found in many places, and now known to owe their 
origin to lightning. Some remarkable tubes of this 
kind were found near Drigg in Cumberland in 1812, 
one of which was considerably more than thirty feet 
in length, and varying in diameter from an inch and 
a half, at the surface of the sandhill in which it was 
found, to half an inch at the bottom of the excavation 
made. Small branches, two or three inches long, 
and a quarter of an inch in diameter, protruded from 
the main stem. The outer surface of these tubes 
was rough and uneven, but the inner surface was 
formed of a "whitish or limpid vitrified matter, 
covered with a smooth glaze, and hard enough to 
scratch glass." 1 Similar tubes were found by Darwin 
in South America near the Eiver Plata, 2 and attempts 

1 The Thunderstorm, p. 117. 
2 Journal during the Voyage of the Beagle, pp. 43, 44. 



86 STATIC ELECTRICITY parti 

have been made with partial success to imitate them 
artificially, by means of passing a powerful electric 
discharge through various hard, powdered substances. 
The experiments succeeded with glass dust, a tube 
an inch long having been formed in this way, but 
failed with felspar and quartz, out of which lightning 
has nevertheless manufactured tubes thirty feet in 
length. 

The most remarkable of all the effects due to 
lightning, however, are the extraordinary explosions 
it causes, and which have been attributed to the 
sudden vaporisation of any moisture contained in the 
solid materials, such as stone, wood, etc., through 
which the discharge is passing. The formidable 
expansive power of steam is well known, and we 
may therefore conceive that if all the moisture con- 
tained in a tree or in a mass of stone were suddenly 
turned into vapour, the pressure would be such as to 
burst everything before it. 1 Instances of explosive 
effects in buildings will be given hereafter. With 
regard to trees, it is stated that " on the 25th of May 
1842, at the village of Adforton, near Ludlow, 
lightning struck a poplar-tree nearly forty-five feet 
high ; it was shivered to pieces, and the ground for a 
hundred yards round it was thickly covered with 
splinters, from four to twelve inches long, many of 

1 Arago's theory as quoted in The Thunderstorm, p. 122. 



chap, vi ATMOSPHERIC ELECTRICITY 87 

which seemed to be entirely smashed. The body of 
the tree was divided into eight or ten large portions, 
which came away with the branches and fell wide of 
each other, but all on the south side." * This is one 
example out of many equally remarkable. 2 Still 
more striking are some of the explosive accidents 
due to lightning on board ship. On 19th September 
1812 H.M.S. Sultan (of seventy-four guns) was struck 
by lightning off the coast of Sardinia, " the highest 
spar or top-gallant and royal mast was fairly shaken 
in pieces ; the next, or topmast, seventy feet long, 
was burst into shreds like a bundle of laths, and 
stood gaping open in the upper end ; it remained in 
this condition for some minutes, and then fell with a 
terrific crash. So complete was the destruction, that 
the decks of the ship were filled with the chips of 
wreck of more than three tons of wood. The next, 
or lower mast, weighing eighteen tons, was struck 
through to the very centre ; and the lightning made 

1 The Thunderstorm, p. 127. 

2 A very similar but not quite so destructive flash is mentioned 
in The Electrician for 27th June of the present year. During a storm 
at Playford in Suffolk, "a poplar tree, about 300 yards away from 
the church, was struck by lightning, and the bark was completely 
stripped away from top to bottom, the southern half of the body 
being riven into matchwood. One piece, 5J lbs. in weight, was 
picked up 126 yards away from the tree, and the debris covered 
about two acres of land. The discharge left the tree at the foot, 
following the direction of a fence for about 15 or 20 feet, threw up 
a sod about a foot square, and went to earth." 



88 STATIC ELECTRICITY part i 

one or two holes in it sufficiently large for a boy to 
creep into. The chips which were torn out were 
thrown about the deck. It was with difficulty pre- 
vented from falling till the ship got into port, when 
the mast in its ruined state was taken out. On 
removing the mouldings and fishes it literally fell to 
pieces." Y 

The length of lightning flashes has been very 
variously computed, some authorities considering that 
it may be a mile or even more in length, others that 
it can never exceed 500 or 700 feet. 2 Probably the 
truth lies somewhere between these two extremes. 
At any rate it seems certain that thunderclouds never 
attain very high altitudes, probably not over 7000 
feet above the sea-level. On high mountains thunder- 
clouds are seen below the observer; and thus it would 
appear that while aurorse are chiefly, if not entirely, 
confined to the upper strata of the atmosphere, 
thunderstorms take place in the lower, 

The magnetising power of lightning must not be 
left unmentioned. After thunderstorms at sea the 
action of the ship's compass has often been impaired. 
Occasionally the polarity of the needle is actually 
reversed, and often pieces of iron and steel become 

1 The Thunderstorm, p. 130. 

2 See discussion on lightning-rods at the meeting of the British 
Association, 1888. 



chap, vi ATMOSPHERIC ELECTRICITY 89 

magnetic through the passage of lightning. "Watch- 
springs have frequently suffered in this way, and 
after the remarkable storm during which the church 
of St. George's, Leicester, was partially destroyed, 
described in the ensuing chapter, the iron cramps in 
the steeple and other masses of metal were found to 
be highly magnetised. These magnetising effects 
are taken to indicate that the duration of lightning 
is, at any rate occasionally, considerably longer than is 
usually supposed, as magnetisation is not momentary, 
but takes an appreciable time to accomplish. 

Thunderstorms are not the only occasions of the 
manifestation of lightning. Violent volcanic erup- 
tions are always accompanied by this, and sometimes 
by other electrical phenomena. Lightning seems to 
have been a specially-marked feature of the terrible 
Krakatoa convulsion, nearly all eye-witnesses of the 
eruption laying stress upon this, as greatly adding to 
the horror and magnificence of the spectacle. Thus 
Captain Woolridge of the Sir R. Sale, viewing the 
volcano from the N.E. at sunset on Sunday evening 
26th May, describes the sky as presenting "a most 
terrible appearance, the dense mass of clouds being 
covered with a murky tinge with fierce flashes of 
lightning. At 7 p.m., when the dense vapour and 
dust clouds rendered it intensely dark, the whole 
scene was lighted up from time to time by the 



90 STATIC ELECTRICITY parti 

electrical discharges, and at one time the cloud 
above the mountain presented the appearance of an 
immense pine-tree, with the stem and branches 
formed with volcanic lightning." l The same ob- 
server, at a distance of forty miles, speaks of the 
great vapour cloud looking like " an immense wall, 
with bursts of forked lightning, at times like huge 
serpents, rushing through the air." Another observer, 
whose vessel was situated about forty or fifty English 
miles N.W. of the volcano, records that " lightning 
struck the mainmast conductor five or six times," 
and also that "the mud rain covering the masts, 
rigging, and decks was phosphorescent ; the rigging 
presenting the appearance of St. Elmo's fire." 

"This abundant generation of atmospheric elec- 
tricity," writes Professor Judd, " is a familiar phenom- 
enon in all volcanic eruptions on a grand scale. 
Steam jets rushing through the orifices of the earth's 
crust constitute an enormous hydro-electric engine ; 
and the friction of ejected materials striking against 
one another in their ascent and descent also does 
much in the way of generating electricity." 2 

1 Official Report of the Royal Society on the Eruption of Krakatoa, 
p. 19. 
2 Ibid. p. 21, 



CHAPTEE VII 

ATMOSPHERIC ELECTRICITY — continued 

DANGERS TO BE APPREHENDED FROM LIGHTNING 

MODES OF PROTECTION 

Danger to life and property from lightning — Instances of loss of 
animal life — Death not inevitable from a "stroke" of lightning 
— Persons struck do not see the flash — Instance — Globular 
lightning is seen — Case of Mr. Pitcairn — Injury to buildings — 
Account of the partial destruction of St. George's Church, 
Leicester — Dangers formerly encountered at sea during thunder- 
storms — Examples — Explosion of gunpowder magazines by 
lightning — Earliest lightning conductors — Their inefficiency — 
System of protection devised by Sir W. Snow Harris — Direc- 
tions concerning lightning-rods — Modern modifications — Real 
use of lightning-rods — Perfect system of protection not yet 
attained — Necessity of employing experienced electricians to 
erect lightning-rods — Protection of individuals from lightning 
— Metal does not "attract" lightning — Theory of an "area 
of protection" exploded — The most elevated objects not always 
those struck — Reason. 

The dangers to be apprehended from lightning are, in 
the case of property, destruction by fire, or by the 
extraordinary explosive powers sometimes mani- 



92 STATIC ELECTRICITY parti 

fested ; and in the case of man and the lower 
animals, severe injury or death, owing to the shock 
to the nervous system, 1 caused by the passage through 
a living body, of electricity in such large quantities 
and at such high potential. Timid persons are often 
ridiculed for the terror they show during thunder- 
storms. There is, however, something to be said in 
their defence, for though all storms are not dangerous, 
there is no doubt that some are so in a high degree, 
and moreover the disturbed electrical condition of 
the atmosphere before and during a heavy storm, is 
of itself sufficient to induce great nervous discomfort 
in sensitive organisations. Accidents due to light- 
ning are now made so public, owing to the news- 
papers which flood the civilised world, that they 
perhaps appear more frequent than they really 
are ; but it is only necessary to remember the 
large number of thunderstorms which annually 
take place in the United Kingdom, and the 
small average of deaths either of men or animals 
occasioned by them, to feel reassured on this 
point. Before the introduction and better under- 
standing of lightning-rods, indeed, high isolated 

1 This was until recently the reason assigned for death from 
lightning, or any powerful electric "shock." It appears, however, 
according to the latest investigations, that some direct effect is 
produced upon the heart, and that is most likely the immediate 
cause of death. 



chap, vii ATMOSPHERIC ELECTRICITY 93 

buildings, particularly churches, were only too often 
"struck," and ships also suffered terribly, but 
in our days such occurrences are comparatively 
rare, and it is to be hoped that with the further 
advance of electrical knowledge they will cease 
altogether. 

Before entering on the subject of protection 
from lightning, it may be interesting to give a few 
examples of the terrible destruction it may work. 
First with regard to animal life. In 1858 twenty- 
five sheep were killed during a thunderstorm near 
Abingdon. 1 In the same year at Sacco in Italy, on 
the 17th of August, 120 sheep out of a flock of 140 
were killed by lightning. The shepherd and the 
shepherd's boy were not injured, though the latter 
was carrying a kid in his arms which was killed. 2 
This remarkable escape of human beings, when 
animals close by them are struck, is not unusual, and 
would seem to point to the conclusion that the lower 
animals are more susceptible than man to injuries 
from lightning. In some instances this could be 
accounted for by the greater propensity of animals to 
huddle together in groups, especially when terror- 
stricken. During thunderstorms such a tendency is 
dangerous, because the column of heated air rising 

1 The Thunderstorm, p. 163. 
2 Ibid, p. 163. 



94 STATIC ELECTRICITY part i 

from their bodies offers a good conducting path to 
the electric discharge. No such cause, however, 
could have accounted for the death of the kid in the 
shepherd-boy's arms, and the escape of the boy him- 
self. When human beings are struck and recover, 
(for to be struck by lightning by no means neces- 
sarily entails death,) they invariably say they did 
not see the flash which rendered them unconscious. 
Many instances of this are cited in The Thunderstorm, 
to which work frequent reference has already been 
made. A relative of the present writer, who was 
struck by a flash of lightning during a violent 
thunderstorm in Yorkshire in 1886, saw nothing, 
neither did his wife, who was close to him and 
suffered, though slightly in comparison, from the 
effects of the same flash. More curiously, the woman 
in whose farmhouse they had taken temporary refuge, 
and who was in another room, saw T no flash either at 
this moment, but heard a sudden sharp explosion 
which she took for the report of a gun, and rushed 
in a state of great anger to demand an explanation 
from her guests, when, to her astonishment, she found 
one of them in an unconscious and apparently dying 
state. A short time sufficed to restore his senses, 
and no other injury was sustained, beyond a shock to 
the nervous system, which for a time affected the 
general health and spirits, but not in any serious or 



CHAP, vii ATMOSPHERIC ELECTRICITY 95 

incapacitating manner. 1 Globular lightning, whose 
movements are so much slower, is not thus invisible 
to those whom it injures. M. Plante cites an instance 
of a ball of fire the size of a fist, which, during a 
violent thunderstorm, appeared to two clergymen, 
Messrs. Wainhouse and Pitcairn, who were together 
in a room in the rectory of Steeple-Ashton, Wilt- 
shire. It seemed to be one foot distant from them, 
and about the height of a man from the ground, and 
was surrounded by a dark smoke. Its explosion was 
attended by a noise comparable to the firing of several 
cannons and a strong a sulphurous " smell filled the 
house immediately afterwards. Mr. Pitcairn was 
dangerously wounded. 2 

With regard to buildings, churches and any high 

1 One of the curious tree-like marks which have often been 
noticed on the bodies of persons struck by lightning, was produced 
in this instance, and remained for a considerable time. It extended 
over the lower ribs on the right side, and resembled the trunks of 
two trees close together, with branches ramifying from them. The 
sufferer on this occasion also said that though he did not see the 
lightning, he heard the thunder ; in fact, it was the last thing he 
remembered before losing consciousness. 

2 Phenomenes eledriques de V Atmosphere, p. 221. In a previous 
portion of this work M. Plante justly remarks "that it is not a 
small mass of air rarefied and rendered luminous by the passage of 
an electric current which could thus explode with the noise of 
thunder and resolve itself into f strokes ' of lightning. The source 
of this final phenomenon is in the reservoir of electricity contained 
in the thundercloud, which discharges itself at the point where the 
first escape began in the form of a ball of fire," p. 50. 



96 STATIC ELECTRICITY parti 

or isolated erections are most exposed to danger from 
lightning. Mr. Tomlinson gives a formidable list of 
damages in The Thunderstorm, occurring between the 
years 1822 and 1858, in which no fewer than thirty- 
three churches are included, and enters into many 
interesting details with regard to several of them. 
The most striking account is that of the injuries 
sustained by St. George's Church, Leicester, on 
1st August 1846, during a storm of quite exceptional 
violence and duration, and in which the phenomenon 
of globular lightning repeatedly presented itself. 
The storm had already been raging for hours accom- 
panied by torrents of rain, when "at five minutes 
past eight, after one or two peals of unusual distinct- 
ness, the Church of St. George was struck with a 
report resembling the discharge of cannon, and with 
a concussion of the air which shook the neighbouring 
houses, and extinguished a lamp burning at the 
entrance of the newsroom, many hundred feet 
distant. . . . Two of the spectators of this awful 
event were Captain Jackson and the Eev. E. Burnaby, 
rector of the parish, who both described the flash as 
a vivid stream of light, followed by a red and 
globular mass of fire, and darting obliquely from the 
north-west with immense velocity against the upper 
part of the spire. For the distance of 40 feet on 
the eastern side, and nearly 70 on the west, the 



chap. Til ATMOSPHERIC ELECTRICITY 97 

massive stonework of the spire was instantly rent 
asunder and laid in ruins. Large blocks of stone 
were hurled in all directions, broken into small 
fragments, and in some cases, as there is every reason 
to believe, reduced to powder. One fragment of 
considerable size was hurled against the window 
of a house 300 feet distant, shattering to pieces 
the woodwork, as well as fourteen out of the 
sixteen panes of glass. ... It has been computed 
that a hundred tons of stone were on this occasion 
blown to a distance of 30 feet in three seconds. 
In addition to the shivering of the spire, the pinnacles 
at the angles of the tower were all more or less 
damaged, the flying buttresses cracked through and 
violently shaken, many of the open battlements at 
the base of the spire knocked away, the roof of the 
church completely riddled, the roofs of the side 
entrances destroyed, and the stone staircases of the 
gallery shattered. The top of the spire, when left 
without support beneath, fell perpendicularly down- 
wards inside the steeple, causing much devastation in 
its descent." 1 

The scene of this fearful accident 2 was afterwards 

1 The Thunderstorm, p. 153 ff. 

2 One almost equally severe appears to have occurred at Louvain 
on 8th April of the present year 1890. The cathedral was struck by 
lightning. "One of the turrets was completely destroyed, and the 
top, weighing about four tons, was projected a distance of twenty- 



98 STATIC ELECTRICITY part i 

minutely examined, when it became evident that the 
formidable explosions which worked the destruction, 
were caused by the electric discharge on its road to 
the earth, bursting its way from one good conducting 
point to another through masses of badly conducting 
material. Thus we are told that "after traversing 
the vane and spindle, and the terminating iron 
supports, the only path left for the fluid was 
through a series of iron cramps, separated by means 
of sandstone; and here it was that the explosion 
commenced, — the stone being torn and hurled aside as 
it came in the path of the lightning to the lowest 
lead lights of the spire." l 

In this accident, terrible as it was, no life was 
sacrificed, but M. Plante cites an account of a violent 
storm which took place on the 27th of July 1769, 
during which several hundred persons being congre- 
gated in a large public hall suddenly saw a fiery 
globe the size of a large cannon ball appear through 
an opening in the roof. All the lights immediately 
went out, and more than seventy-six persons were 
killed or wounded. 2 

two yards, demolishing a house, while blocks of stones weighing 
from two to three tons were hurled a distance of nearly seventy 
yards, damaging the houses in the neighbourhood." — The Elec- 
trician for 6th June 1890, p. 109. 

1 The Thunderstorm^ p. 156. Apparently this church was 
unprotected by any lightning conductors whatever. 

2 Phenomenes electriques de V Atmosphere, p. 222. 



chap, vii A TMOSPHERIC ELECTRICITY 99 

However fearful the destruction worked by light- 
ning inland may sometimes have been, ships were 
formerly exposed to a far greater degree of danger 
from this source. Isolated objects on the vast plain 
of the ocean, it was almost impossible that during 
thunderstorms, they could escape being made part 
of the path of an electric discharge between the 
clouds and the sea, and the wonder is rather that 
so many escaped, than that so many were " struck." 
Nevertheless, the destruction both of life and pro- 
perty before Sir William Snow Harris devised an 
efficient system of protection, was truly appalling. 
The accidents to H.M. ships alone between the 
years 1790 and 1840 numbered 280, some exceedingly 
serious, and the loss of life was proportionately great, 
100 seamen having been killed, and 250 dangerously 
injured, while the monetary loss to the country 
was reckoned at flSO^OO. 1 Space forbids the 
citation of more than one instance, that of the 
Repulse a 74-gun ship. On the 13th of April 
1810, the Repulse being off the coast of Spain, 
was overtaken by "a heavy squall of wind, with 
rain, thunder, and lightning, at which time the people 
were employed in getting down their w T ashed clothes 
which hung from the rigging, when the ship was 
struck by two vivid flashes of lightning which 

1 The Thunderstorm, p. 171. 
8 



100 



STATIC ELECTRICITY 



PART I 



shivered the maintop -gallant mast, and severely 
damaged the mainmast. Seven men were killed on 
the spot, three others only survived a few days, and 
ten were maimed for life. After the second discharge 
the rain fell in torrents ; the ship was more com- 
pletely crippled than if she had been in action, and 
the squadron then engaged on a critical service lost 
for a time one of its fastest and best ships." * 

More terrible yet is the description of accidents 
which occur when lightning sets a ship on fire. 
Fearful stories of suffering and privation have thus 
been added to the roll of disasters at sea. Amongst 
others the case of the Tanjore, a ship'belonging to 
the East India Company is cited by Mr. Tomlinson. 
In May 1820 she was struck by lightning forty 
miles off the coast of Ceylon, two men were instantly 
killed, and many others rendered insensible ; the 
cargo, which was partly of brandy, caught fire, and 
burned so fiercely that the crew and passengers had 
to hurry into the boats without waiting even to take 
food and water. Fortunately a few hours afterwards 
they met with a native vessel and so were rescued. 2 

Many instances have occurred of powder maga- 
zines being struck and exploded by lightning. " In 
1855, on the 7th of October, about 2 P.M., a firework 
manufactory in Green Street, Liverpool, was struck 

1 The Thunderstorm, p. 172. 2 Ibid. p. 182. 



chap, vii ATMOSPHERIC ELECTRICITY 101 

by lightning and blown up ; the factory and the 
adjoining houses were destroyed, and many persons 
severely injured. ... On the 10th of August 1857, 
about midnight, lightning fell on the magazine of 
Joudpore, in the Bombay Presidency^ whereby some 
thousands of maunds of gunpowder were blown up. 
Five hundred houses were destroyed, and nearly one 
thousand persons are reported to have been 
killed." l 

Enough examples have now been quoted to show 
the need of protection from lightning, and it is time 
to turn our attention to the means employed for 
attaining this end. Lightning conductors w 7 ere an 
almost immediate consequence of Franklin's famous 
experiment in 1752 (described on p. 58 of the present 
work), and only ten years later the first erected in 
England was put up by Dr. Watson at Paynsted. 
It is amusing to think that so little was the principle 
of their action understood, that during the War of 
Independence an animated discussion was carried on 
between the supporters of pointed lightning - rods 
and those who recommended rounded tops. The 
matter was made a political question, the pointed 
rods being in favour with Franklin and the Eevolu- 
tionary party, and the blunt with " loyal subjects and 
good citizens ; " 2 neither side considering the scien- 

1 The Thunderstorm, p. 169. 2 Ibid. p. 223, note. 



102 STATIC ELECTRICITY part i 

tific and practical question as to which was in reality 
the best protector worth attending to ! As a matter 
of fact, the pointed rods were of course preferable, 
points contributing to a silent and noiseless discharge, 
and round tops, on the contrary, being likely to cause 
an explosive discharge between the conductor and 
the cloud. In these early days, however, many other 
important matters besides that of points, were but 
ill understood. One lightning-rod stuck up any- 
where, perhaps insulated at the bottom, instead of 
having a good earth connection, or run into a small 
stone tank of water, or into any other equally im- 
possible place according to the better instructed 
ideas of the present day, was deemed sufficient. 
Ships also were provided with a chain conductor of 
very small dimensions, stowed away in a box, and 
taken out to be suspended from the masts if occasion 
required ! It is not surprising that such protection 
as this was found terribly inadequate, and that great 
danger was incurred by the sailors, through having 
to place these conductors in position during a storm. 
To Sir William Snow Harris belongs the honour of 
having first devised a really adequate system of pro- 
tection both for buildings and ships. He insisted 
upon an unbroken line of metallic connection between 
every part of the building or vessel and the light- 
ning-rods as indispensable. Isolated masses of metal 



chap, vii ATMOSPHERIC ELECTRICITY 103 

forming an integral part of any erection are fraught 
with danger, and exceedingly likely to cause such 
destructive explosions as those which occurred in 
the case of St. George's Church, Leicester, cited 
above, — for the lightning in leaping from one con- 
ducting point to another will shatter the badly- con- 
ducting substances obstructing its path. The upright 
rod or rods should be armed with one or more points, 
and should project above those portions of the build- 
ing to which they are attached. They should also 
have a thoroughly good earth connection, i.e. the 
earth about them should be kept damp, and they 
should not be buried in charcoal, or beds of stone or 
rubble, nor led into enclosed tanks. 1 Running water 
on the contrary is excellent, as it affords a thoroughly 
good conducting channel for the electric discharge. 

Lightning-rods should also have a good extent of 
surface, but they need not, as was formerly thought, 
be solid. Hollow rods are quite as good, and flat 
ribbons, or a bundle of separate strands of thickish 
wire, better still. The solid rods were used under 
the idea that they offered a greater amount of con- 
ducting material to the passage of the electric dis- 
charge, because currents of electricity (and a discharge 
is a momentary current,) penetrate the substance of 

1 It is specially recommended also that they should be buried 
deeply in the earth, not too near the surface. 



104 STATIC ELECTRICITY part i 

conductors, and do not remain on the surface like 
static charges ; but this only holds good of steady 
currents, not of sudden rushes of electricity like 
the discharge of a Leyden jar and lightning. In 
these cases the velocity is so great and the duration 
so short, that the electricity scarcely penetrates below 
the surface, and therefore the important matter is 
that the latter should be of sufficient extent, not that 
the conductor should be solid. A modification has 
also arisen in the views held as to the best metal to 
employ in the construction of lightning-rods. Until 
recently, copper was recommended by all electricians 
on account of its high conducting power, but it 
appears now that for this very reason it may be less 
suitable than iron, because, as Dr. Oliver Lodge 
stated in a lecture delivered before the Society of 
Arts on 17th March 1888, " If a great weight or a 
large reservoir of water were propped up above one's 
house, one would not say that, the safe thing being 
to get it down as quickly as possible, it was advisable 
to break away the props, or to blow the bottom out 
of the reservoir; no, one would prefer to let it slide 
slowly and gradually down a well-resisting channel, 
so as to disperse the energy gradually." 

These words " to disperse the energy gradually " 
recall a consideration of the highest importance 
which must never be lost sight of jn any system of 



chap, vii ATMOSPHERIC ELECTRICITY 105 

protection from lightning. The question at issue is 
not only — or even mainly — how to conduct a certain 
quantity of electricity safely and quietly into the 
earth, but how to dispose of the enormous energy 
developed by lightning which can neither be ignored 
nor conjured out of existence. Therefore a certain 
amount of resistance may be a good thing as afford- 
ing work to do to overcome it, but it need hardly be 
said that it should never be sufficient to occasion great 
heating, as that at once entails the danger of fire, or 
the collapse of the conductor through fusion, either 
throughout its length, or at special points, such as 
the joints, where increase of resistance is encountered. 
Even with every precaution taken, and a system 
of protection from lightning adopted in accordance 
with the best practical electrical experience of the 
day, absolute safety cannot be guaranteed, as was 
abundantly proved by the Hotel de Ville at Brussels, 
which is protected by a most elaborate and care- 
fully -carried -out system of lightning conductors, 
having been struck by lightning, and a portion set 
on fire in the month of June 1888. But little 
damage was done, as the fire was almost immedi- 
ately extinguished. Nevertheless the case is a most 
important and instructive one, showing as it does 
that protection from lightning, though vastly im- 
proved, is not even yet perfectly understood. One of 



io6 STATIC ELECTRICITY parti 

our leading electricians distinguishes between two 
main cases of lightning flash, the one caused by a 
steadily-increasing strain between a cloud and the 
earth, so that the path of the flash is inductively 
prepared beforehand, the other by a sudden rise of 
potential in a cloud, between which and the earth 
no strain previously existed, by the discharge of 
another cloud into it, so that an " impulsive rush " 
of electricity takes place to the earth without any 
previous preparation. He considers that in the 
first of these cases, a system of protection carried 
out according to present ideas would be efficient, 
but not in the second case, and his conclusions are 
based upon a number of interesting and highly 
instructive original experiments. 1 

The foregoing remarks, though giving the merest 
outline of the subject, may nevertheless enable the 
general reader to understand something of its im- 
portance, and will at any rate serve to show him 
that the protection of a building from lightning can- 
not possibly be properly accomplished by any but 
practical electricians. A village workman must not 
be depended on for the erection of lightning-rods. 

1 See Mann Lectures before the Society of Arts by Dr. Oliver 
Lodge, F.R.S., in March 1888 ; also a paper by the same author on 
"Lightning, Lightning Conductors, and Lightning Protectors," 
read before the Institution of Electrical Engineers in April 1889, 
reported m The Electrician of 3d May 1889. 



chap, vir ATMOSPHERIC ELECTRICITY 107 

It may perhaps be of interest to mention that in the 
case of gunpowder magazines and other stores of 
explosive material, high pointed conductors are not 
recommended. It must always be remembered that 
owing to the facilities they offer, they are likely, if 
present in sufficient numbers and at a sufficient alti- 
tude, to determine a discharge, which might not 
otherwise take place, between a cloud and the earth, 
and though in ordinary cases this would not be a 
source of danger, it certainly would where gunpowder 
or other explosive substances are concerned, as the 
smallest side spark (such as frequently takes place 
from lightning-rods to other conductors in close 
proximity,) might cause a terrible accident. A net- 
work of iron entirely covering the edifice, or better 
still, making the erection itself of iron, is far prefer- 
able in such cases. 1 

1 Telegraphic and telephonic instruments and stations require 
special protection from lightning, and the guards with which they 
are provided, are almost always constructed on the principle that 
owing to the "impedance " ottered by good conductors to a sudden 
flash (due to the oscillatory nature of the latter), it would rather 
jump over a short air-space than follow a length of wire. Double 
combs are used as protectors in telephone exchanges ; a pair of 
plates separated by a very short space in telegraphic offices. These 
do not always prove efficient, however, and Dr. Oliver Lodge has 
devised a new lightning guard, for which he claims almost absolute 
perfection, and whose principle, in his own words, consists in 
taking "the overflow from one protector and giving it the chance 
of another ; then taking the overflow from this and offering it 
another air-gap, and so on till nothing is left ; at the same time 



108 STATIC ELECTRICITY tart i 

"With respect to the protection of individuals from 
lightning, a few plain directions may be given. It is 
a well-known source of danger to stand close under 
trees or under any high and isolated object. Detached 
pieces of metal worn about the person should also be 
avoided, "as well as standing near a fireplace if there 
is a fire burning. It is of no use to cover oneself 
with silk garments, or in fact to attempt insulation 
in any way, as this only increases danger. On the 
other hand, the rather impossible protection of a suit 
of armour would render its wearer perfectly safe 
so long as the joints did not become overheated, 
which might perhaps occur if the armour were 
actually struck. Such a defence, however, though 
excellent for the* owner, would be exceedingly dan- 
gerous to his friends, for to touch it during the pro- 
gress of a storm would ensure a violent, possibly 
fatal shock ; and the same remark applies to iron 
network over houses and other erections. The build- 
ing thus enclosed and all its inmates would be per- 
fectly safe, but any one approaching it from the 
outside during a storm, and laying his hand upon the 
metal, would certainly rue the consequences. 

diminishing the overflow from each protector as much as possible 
by the use of self-induction coils, which impede the violently vary- 
ing or alternating rushes by their electro-magnetic inertia." — Quoted 
from a paper read before the Institution of Electrical Engineers on 
24th April 1890, published in Tine Electrician for 23d May. 



chap, vii ATMOSPHERIC ELECTRICITY 109 

It must not be supposed, however, that there is 
anything in metal which attracts lightning. Such is 
not the case. On account of its high conducting 
power, it offers an easy path to the electric discharge, 
of which the latter will, if possible, avail itself ; but 
it will not go out of its way to pick out a lightning- 
rod, or any other metallic conductor. Instances 
have occurred of lightning striking buildings in close 
proximity to lightning-rods, erected under the idea 
that they would afford an " area of protection," within 
the limits of which nothing but themselves could be 
struck. This erroneous theory of an "area of pro- 
tection " is fast dying out, as also that of the most 
elevated objects being always the ones struck. Very 
many instances are on record of houses in the imme- 
diate neighbourhood of tall trees suffering from 
lightning, while the trees themselves escaped ; and 
this, though apparently surprising at first, ceases to 
be so when we remember, that the state of strain to 
which all the phenomena of discharge are due, does 
not exist primarily between a cloud and the lightning- 
rod, tree, or steeple, or whatever the elevated object 
may be, but between the cloud and the whole of that 
portion of the earth's surface lying beneath it ; there- 
fore all that these isolated and, in comparison to the 
extent of surface, small elevations can do, if they are 
good conductors, is to protect themselves from danger 



no 



STATIC ELECTRICITY 



PART I 



by rendering that part of the discharge which is 
taking place between them and the cloud harmless. 
They cannot do more than this, and hence the para- 
mount importance of a metallic connection between 
every part of the building to be protected and its 
lightning-rods. During the discussion on lightning- 
rods which took place between Sections A and G, 
at the meeting of the British Association in Bath 
1888, a most remarkable photograph of a flash 
which occurred during a storm in America was 
shown. It can only be described by saying that 
the sky seemed to be literally pouring down the 
electric fire on every side, and the remark was justly 
made by its exhibitor, 1 " Where in such a case could 
the c area of protection ' be ? " The only real safety 
would lie in the whole surface upon which this enor- 
mous quantity of electricity was descending being of 
good conducting matter; and in the presence of such 
tremendous manifestations of natural energy as this, 
we can but feel that though it behoves us to take 
every precaution which the most advanced Science 
recommends, our preservation depends on a Higher 
Power and a vaster Knowledge than any which our 
resources can command. 

1 The Hon. Kalph Abercrombie, F.R.S. 



PART II 
MAGNETISM 

CHAPTEE I 

GENERAL PROPERTIES OF MAGNETS 

Ancient knowledge of natural magnets — Lodestone an ore of iron — 
Made useful in navigation in the twelfth century — Gilbert's 
discovery of magnetic poles — Attraction between unlike and 
repulsion between like magnetic poles — The earth a magnet 
— Naming of the poles — Magnetic substances — Dia- magnetic 
substances — Action and reaction between magnets and magnetic 
substances equal — Similarities and dissimilarities between elec- 
tric and magnetic induction — Magnetic induction cannot take 
place across magnetic substances — Acts across a vacuum — 
Various ways of making magnets — Consequent poles — Bar and 
horse-shoe magnets — Magnetic shell — Strength and lifting- 
power of magnet — Causes of loss of magnetisation — Subdivi- 
sion of magnets — Molecular theory of magnetism. 

Natural magnets were known from very ancient 
times, and their name is derived from Magnesia, in 
Asia Minor, where many of the hard black stones 
possessing the property of attracting iron and steel 



1 1 2 MA GNE TISM part ii 

were found. The English name of lodestone means 
simply leading stone. It is an ore of iron called by 
mineralogists Magnetite, and exists in large quantities 
in Sweden, Spain, and various other countries, 
though it by no means always possesses magnetic 
properties, being often found entirely destitute of 
them, nor is it known by what means they are 
acquired by those specimens of the ore which 
exhibit them. About the twelfth century it be- 
came known in Europe that lodestones, whose 
power of attracting iron was already attributed 
to magic, possessed another yet more marvellous 
peculiarity, viz. that of setting themselves always in 
a north and south direction when freely suspended. 
This property was made useful in navigation ; and in 
fact the name lodestone is derived from the fact of 
the magnet stones being able to act as mariners' 
guides. The first artificial magnets were made by 
rubbing iron or steel with lodestone, when it was 
found that the latter imparted its magnetic properties 
to these substances, — the iron, however, only retaining 
its magnetism for a very short time, whereas the steel, 
though not able to be so powerfully magnetised as iron, 
did not again lose the properties it had acquired, but 
became a permanent magnet. Other ways of making 
artificial magnets, which will be mentioned in due 
course are now known and extensively used. 



chap. I GENERAL PROPERTIES OF MAGNETS 113 

Dr. Gilbert, whose electrical discoveries have 
already been mentioned, made many of equal import- 
ance respecting magnets, and in his work De Magnete 
described a number of elementary facts regarding 
them. He was the first to notice that the attractive 
power appears to reside at the two ends of a magnet, 
called always its poles} This fact can easily be proved 
by placing an ordinary bar magnet among a number 
of iron filings, which will be seen to arrange them- 
selves in thick tufts round the poles, thinning as the 
centre is approached, while at the actual centre there 
are none. This non-attractive part of the magnet 
Gilbert named the equator, and the imaginary line 
joining the poles, the axis. 

It has already been mentioned that a freely-sus- 
pended magnet sets itself in a particular direction with 
regard to the earth, viz. with one pole pointing nearly 
north, and the other nearly south; moreover, it is 
always the same pole which points in the same 
direction, for if a magnet be turned by any means 
out of its natural position with regard to the earth, 
it will return to it again, the moment the constraining 
force ceases, the pole which was before pointing 
north resuming the same direction. Let it now be 

1 It is only in a long thin bar magnet that the poles are actually- 
situated at the extreme ends, however. In thicker magnets they 
lie slightly nearer the centre. 



ii4. MA GNE T1SM paet ii 

supposed that there are two magnets, the north-seek- 
ing poles of which have both been marked. One of 
these magnets is freely suspended (or balanced upon 
a pivot, which comes to the same thing), the other 
being held in the hand. The free magnet will of 
course be turned into its usual north and south 
direction, and this being so, let its north -seeking 
pole be approached to the north -seeking pole of 
the second magnet, when it will be seen that the 
former is instantly turned away from the latter, 
thus showing that tivo north-seeking poles repel each 
other. 

Let us now vary the experiment by approaching 
the south- seeking pole of the magnet held in the 
hand to the north -seeking pole of the free magnet. 
We shall find that the latter will turn towards the 
south -seeking pole, and if near enough, will rush 
into contact with it, thus remaining till the two are 
separated by force, showing that a north- seeking 
and a south seeking pole attract each other. 

It would appear therefore, that as there are two 
kinds of electricity, positive and negative, so there 
are two kinds of magnetism, north-seeking and south- 
seeking. Moreover, since it is evident from the 
position taken up by a magnet with regard to the 
earth that the north part of the latter attracts one 
pole of the magnet and the south part the other, and 



chap. I GENERAL PROPERTIES OF MAGNETS 115 

since only magnets have this power of attraction 
and repulsion over other magnets, we are driven to 
the conclusion that the earth itself must be a magnet 
obeying the invariable law that like poles repel, and 
unlike poles attract each other, and that this is the 
reason of the north and south direction taken up 
by a freely-suspended magnet, whose north- seeking 
pole points to the south magnetic pole of the earth, 
and its south-seeking pole to the earth's north- 
magnetic pole. It is usual and more convenient to 
employ, instead of a magnet, a magnetic needle for 
experimental purposes. The needle is made of steel 
and is very light and thin, usually ' lozenge-shaped, 
and balanced on a pivot in the manner of that used 
in the ordinary pocket- compass. It is magnetised 
by being rubbed with a magnet ; and has, of course, 
a north -seeking and south -seeking pole. These are 
in common parlance called the North and South 
Poles, at least in England; but the custom leads to 
very great confusion of ideas ; for if we name that 
pole of the magnetic needle pointing towards the 
north magnetic pole of the earth the North Pole, 
and that pointing towards the south magnetic pole, 
the South Pole, we virtually state that like poles 
attract each other, which is the very reverse of the 
fact, and consequently the terms north-seeking and 
south- seeking which have frequently been adopted 



1 1 6 MA GNE TISM part ii 

by English men of science are far more correct, and 
will be used in the present work. 

A magnet always has two poles, one north-seeking 
and one south -seeking; it is quite impossible to 
obtain a magnet with one pole only; but a magnetic 
substance, viz. a substance which, like iron, has the 
power of attracting and being attracted by a magnet, 
has no poles, neither does it appear to have any force 
of repulsion, for it is equally drawn to either pole of 
the magnet which may be presented to it. Iron and 
steel are not the only magnetic substances, nickel 
and cobalt show the same properties, but in a very 
inferior degree, and some other metals, as well as 
paper, porcelain, and oxygen gas are feebly attracted 
if exposed to the influence of a very powerful 
magnet. 

It was for a long time supposed that those sub- 
stances not attracted by a magnet were not influenced 
by it at all, but experiment has proved that if sub- 
jected to strong magnetic action they are repelled, or 
at least appear to be so. Bismuth possesses this 
property in the most marked degree, and a small 
bar of bismuth suspended between the poles of 
two powerful magnets, 1 turns itself so as to lie 
at right angles to the line between the poles, thus 

1 Electro-magnets, which will be described in a future chapter, 
are employed in these experiments. 



chap. I GENERAL PROPERTIES OF MAGNETS 117 

getting as far away from them as possible. 1 A 
bar of iron or steel suspended in the same manner 
would, on the contrary, turn along this line so as to 
present its ends to the poles, thus approaching them 
as closely as it could. Substances which appear to 
be repelled by magnets are called diama gnetic. 2 

By magnetic force is meant the force with which 
a magnet attracts or repels another magnet, or with 
which it attracts a piece of iron or of any magnetic 
substance. This force decreases with distance, and 
between two magnet poles it is directly propor- 
tional to their strength, and, if they are very small and 
far apart, inversely proportional to the square of the 
distance between them. 

It is easily understood that the attraction between 
two unlike magnet poles is mutual. Each is 
drawn towards the other, but perhaps it is not quite 
so evident that the attraction between a magnet and 
a magnetic substance, a lump of iron for instance, is 
also mutual. At any rate, the popular idea is that 

1 This behaviour of the bismuth and of other diamagnetic sub- 
stances has, however, been shown to be due, not to their being 
really repelled, but urged into the weakest part of the. magnetic 
field. Substances which are faintly magnetic behave as though 
they were diamagnetic, if immersed in a medium more magnetic 
than themselves. 

2 Thus named because they allow the magnetic forces to act 
across them, wdiich "magnetic," or as they are often called, "para- 
magnetic " substances will not do. 



1 1 8 MA GNE TISM pa rt i I 

the magnet attracts the iron, but we do not hear of 
the iron attracting the magnet. Yet this is equally 
true, for if a magnet be balanced on a piece of cork 
and set floating in a basin of water, and a lump of 
iron be held near the edge of the basin, the magnet 
will immediately move towards it, just as we should 
see the iron move towards the magnet if their 
positious were reversed. Moreover, the action and 
reaction between a magnet and a magnetic substance 
are equal, just as the action and reaction between 
two magnets are equal. The fact is that a magnetic 
substance is one in which a magnet can induce tem- 
porary or permanent magnetism of the opposite kind 
to that of the pole presented to it, and this is the 
cause of the mutual attraction. 

Magnetic induction can only take place in mag- 
netic substances, just as electric induction can only 
take place in conductors ; and here we may remark 
on the great likeness existing between some of the 
fundamental phenomena of electricity and magnetism. 
Electrified bodies can attract and can repel, so can 
magnets. Electrified bodies can induce electricity 
in other bodies ; magnets in like manner can induce 
magnetism. Yet, though there are great similarities, 
there are also great differences. An electrified body 
has no poles ; its power of attraction and repulsion 
resides all over its surface. Moreover, an electrified 



chap, i GENERAL PROPERTIES OF MAGNETS 119 

body electrifying another by contact, electrifies it in 
the same way as itself, and parts with some of its 
own original charge to do so. A magnet behaves 
quite differently, its poles always magnetise in the 
opposite way to their own magnetism, whether by 
contact or by influence, and no magnet ever loses 
any of its magnetism by imparting it to another 
body. It remains quite as powerful after as before 
the operation. Neither can it be said that magnet- 
ism and stationary electric charges show the slightest 
relationship to one another, 1 though, as we shall here- 
after see, electric currents and magnetism appear to 
be very closely connected indeed ; and since current 
and static electricity have been proved to be the same 
agent manifesting itself under different conditions, 
we may justly infer that magnetism is not of a 
wholly different nature from either. 

Magnetic induction will take place through any 
substance, provided it be not itself magnetic. A 
magnet enclosed in glass, or wood, or immersed in 
water will equally exert its power of attraction and re- 
pulsion. But it cannot do this across thick iron, and a 
magnetplaced within a thick hollow iron ball, is incap- 
able of influencing, or being influenced by any outside 

1 It appears that if a magnet and a charged body are in relative 
motion, a very slight interaction occurs, tending to make them 
revolve round each other. 



120 MAGNETISM part ii 

magnet. The reason of this would appear to be that 
the action and reaction between the magnet and the 
iron employs the whole magnetic force of both, and 
therefore none can penetrate beyond the iron. Mag- 
netic induction can take place across a vacuum, thus 
showing that the presence of ordinary matter is not 
necessary to the transmission of the magnetic forces, 
and that the real medium by which they are con- 
veyed is the ether. 

As has already been stated, there are various ways 
of making magnets. The simplest, but not the best, 
is by stroking the bar or needle of steel to be mag- 
netised from end to end with a lodestone or steel 
magnet. This is called magnetisation by single 
touch. Another and better way is to use two mag- 
nets, commencing by placing their opposite poles 
together in the centre of the bar to be magnetised 
(which is laid in a horizontal position), and then 
drawing them along to the ends, repeating the opera- 
tion several times over. This is called magnetisation 
by divided touch. Both sides of the bar should be 
subjected to the same treatment, and care taken 
that it is methodically and regularly followed out, 
otherwise there may be points between the true 
poles where other poles will be formed, called conse- 
quent poles, thus weakening the external influence of 
the magnetthrough the reaction of theconsequent poles 



chap. I GENERAL PROPERTIES OF MAGNETS 121 

on each other. Magnets can be made by the action of 
the earth's magnetism on bars of steel held in the 
magnetic meridian, i.e. with one end directed towards 
the magnetic north, and the other towards the mag- 
netic south pole of the earth, and struck by a wooden 
mallet while in this position. A bar of steel raised 
to a red heat, and allowed to cool while lying in the 
magnetic meridian, also acquires magnetic properties ; 
but by far the most powerful magnets are made by 
placing a bar of steel or iron inside a coil of wire, 
through which an electric current is caused to 
pass. The steel is made into a permanent magnet 
by this operation, but the soft iron only retains the 
whole of its magnetic properties while the current is 
passing. During this time, however, it becomes 
magnetised to a very high degree indeed, and these 
electro-magnets, as they are called, are by far the 
most powerful of any, and will be described and 
explained in a later chapter. Magnets which have 
been magnetised to the highest degree of which they 
are capable, are said to be saturated. 

A magnet can be made of any shape, but the bar 
and the horse-shoe are the most common. Instead of 
a bar a bundle of steel wires may be magnetised, and 
will act as one magnet. These laminated magnets 
are stronger than single bars, provided the wires be 
magnetised separately before being put together. 



122 MAGNETISM part ii 

A thin sheet of metal may be so magnetised that the 
whole of one face of it will possess north-seeking, 
and the whole of the opposite face south -seeking 
magnetism. Such an arrangement is called a 
magnetic shell, and is of considerable interest and 
importance, because it in many respects greatly 
resembles a closed voltaic circuit. 

By the strength of a magnet is meant the amount 
of magnetic force it possesses, i.e. the power of attrac- 
tion and repulsion shown by its poles ; but the lifting- 
power of a magnet is a different thing. It signifies 
the weight which a magnet is able to support, and 
that depends on the surfaces of contact as well as on 
the strength of the magnet. A horse-shoe magnet 
has much more lifting-power than a bar magnet, for 
the simple reason that both its poles are pressed 
into the service, whereas with a bar magnet one 
only can be employed for this purpose. 

Though steel once magnetised becomes so per- 
manently, as we have seen, there are, nevertheless, 
circumstances under which it cannot retain its 
magnetic properties. They are weakened if the steel 
is very much heated, though partially recovered as 
it cools, and lost altogether if it is made red-hot. 
If the ordinary temperature of a steel magnet is 
lowered, on the contrary, its strength increases, unless 
the cold to which it is subjected be very extreme 



chap. I GENERAL PROPERTIES OF MAGNETS 123 

indeed, when it loses its magnetisation. 1 The same 
thing happens if it is roughly used and knocked 
about ; and this seems to point to the conclusion that 
magnetism is closely connected with molecular struc- 
ture. A far more convincing testimony is borne to 
this theory, however, by the effect of rupture on 
a magnet. The latter appears, as has been stated, to 
have no magnetic force at its centre. Nevertheless, 
if a magnet is broken in half, each half will be a 
perfect magnet with a north-seeking and a south- 
seeking pole, and if the halves be broken in their 
turn, four perfect magnets will have taken the place 
of the single original one. In fact, the process may 
be repeated indefinitely, and if the single magnet were 
to be broken a hundred or a thousand times, each 
piece, however small, w 7 ould still be a perfect magnet. 
Only one theory has been put forward which 
seems to give a satisfactory explanation of this 
phenomenon. This is, that every molecule con- 
tained in a magnet is itself an infinitesimal magnet 
with a north- seeking and a south-seeking pole, and 
that the state of magnetisation consists in all the 
molecules being turned the same way — set end to 
end as it were, so that the north-seeking poles all 

1 Professor Silvanus Thompson states that a steel magnet 

brought down to a temperature of 100° C. below zero loses its 

magnetic properties. — Elementary Lessons in Electricity and 
Magnetism, p. 84. 



124 MAGNETISM part n 

point in one direction, and the south-seeking poles 
in the opposite direction. The inevitable result of 
such an arrangement would be, that the magnetic 
force would always appear to lie at the ends of the 
magnet, and yet that it could be divided into any 
number of perfect magnets. 

There are other phenomena besides those already 
mentioned which support the molecular theory of 
magnetisation. A bar of steel when magnetised 
slightly lengthens, thus showing that there is a 
change of arrangement in the molecules, and that 
it must be one which places them parallel to each 
other. Water, rendered muddy by being mixed 
with fine magnetic oxide of iron, becomes, when 
magnetised, clearer in the direction of magnet- 
isation, as though light were able to pass better by 
reason of the parallel arrangement. A metallic clink 
is also heard when iron is magnetised and demagnet- 
ised, and when this is rapidly done it becomes hot, 
showing that internal friction must take place. 1 

The space all round a magnet within which the 
magnetic forces make themselves felt is called a 

1 Since the above was written, Professor Ewing lias given, in a 
paper on magnetic induction, communicated to section A at the 
meeting of the British Association at Leeds, 1890, an account of 
some highly interesting and important experiments, which appear 
to place the molecular theory of magnetization on a much firmer basis 
than has ever been the case before. It may, in fact, be said to be 
to a great extent proved. 



chap. I GENERAL PROPERTIES OF MAGNETS 



125 



magnetic field, and the directions along which these 
forces act have been called lines of force. Their 
shapes differ according to the number, position, and 
shape of the magnets forming the field. 

Figures of these lines of force are obtained bv 
placing a sheet of paper over a magnet, and then 
sifting very fine iron filings through a muslin bag 
over the paper. They arrange themselves in beau- 
tiful curving lines, each particle taking up the posi- 




Fig. 11.— Lines of force in a magnetic field formed "by a single oar magnet. 



tion assigned to it by the combined action of both 
poles, so that at every point of the lines the resultant 
direction of the attractive and renulsive forces is 



126 



MAGNETISM 



PART H 



accurately shown. Fig. 11 gives the curves of the 
lines of force in a magnetic field formed by a single 





Fig. 12.— Lines of force in a magnetic 
field formed by two bar magnets, 
with north-seeking and south-seek- 
ing poles confronting each other. 



Fig. 13. — Lines of force in a magnetic 
field formed by two bar magnets, 
with north-seeking poles confront- 
ing each other. 



bar magnet; Fig. 12 shows those in a field con- 
taining two bar magnets with opposite poles 
confronting each other ; Fig. 13 those where similar 
poles confront each other. These two last figures 
illustrate most strikingly the action of the attrac- 
tive and repulsive forces. The lines from the 
opposite poles which attract each other, curve in- 
wards so as to enter the one pole from the other; 
the lines from the similar poles which repel each 
other curve outwards, turning aside at right angles so 
as to get as far away as possible. 

The size and strength of a magnetic field depend 
on the strength of the magnet or magnets contained 
in it, and on their position with regard to each other. 
The strength is always greatest near the poles. Any 
magnetic substance placed in the field becomes for 
the time magnetised by induction. 



CHAPTEE II 

MAGNETISM OF THE EARTH 

Gilbert discoverer of the earth's magnetism — Oscillations of 
magnetic needle before settling itself in the magnetic meridian 
— Magnetic intensity — Magnetic poles of the earth — Declina- 
tion of the needle — Variations in the declination and their 
causes — Line of no variation — Isogonic lines — Inclination of 
the needle — Inclination compass — Magnetic equator — Isocliuic 
lines — Magnetic maps — Fluctuations in the earth's magnetism 
— Periodicity in the occurrence of magnetic storms — Influence 
of the sun on terrestrial magnetism — Effect of the earth's 
magnetic force on the magnetic needle, directive only — Magnet- 
isation of steel and iron objects by the earth — Magnetism of 
iron ships. 

The honour of first discovering that the earth itself 
is a large magnet belongs to the same Dr. Gilbert 
whose name has already been several times mentioned. 
As we have seen, a magnet or magnetic needle when 
freely suspended, i.e. hung from, or balanced on the 
point, which is its centre of gravity, takes up a 
particular position with regard to the earth, turning 
itself so as to lie in the magnetic meridian. It does 
not do this with one steady movement, but undergoes 



128 MAGNETISM part ii 

a series of oscillations before finally reaching: its 
position of equilibrium, and every time it is moved 
or disturbed the oscillations are repeated. By making 
calculations on the number which occur in one 
minute, the strength of the earth's magnetism, called 
its magnetic force or intensity, at the particular locality 
can be discovered, though it must be remembered 
that the weight, shape, and length of the magnet 
have to be taken into account, the number of oscilla- 
tions executed in a minute depending on them as 
well as on the strength of the earth's magnetism. 

From the fact that the magnetic needle does not 
point due north and south, we may infer what is in 
fact actually known, that the earth's magnetic poles 
do not coincide with its geographical poles. The 
north magnetic pole is situated just within the 
Arctic Circle in Lat. 70° 5' K, Long. 96° 46' W. The 
south magnetic pole has never been discovered, and 
from various indications it is thought that there may 
be two. The angle made by the magnetic needle 
with the geographical meridian is called its declination, 
and this is continually varying. The north-seeking 
pole lies at present to the west in Europe and 
Africa, and to the east in Asia and the greater part 
of North and South America. Some of the variations 
in the declination of the needle take place gradually 
through a number of years, some annually and 



chap, ir MAGNETISM OF THE EARTH 129 

daily, 1 and some are the result of sudden electric 
and magnetic disturbances, such as displays of the 
Aurora Borealis, earthquakes, and volcanic eruptions. 
Thunderstorms and ordinary atmospheric perturba- 
tions produce no such effect, however. These acci- 
dental variations in the declination of the needle are 
known as magnetic storms, and are sometimes very 
marked indeed. Magnetic storms are always attended 
by a display of the Aurora Borealis in northern lati- 
tudes, and are sometimes simultaneous at widely- 
distant places on the earth's surface. 

There are certain parts of the earth where the 
magnetic north and south do actually coincide with 
the geographical north and south, and at these places 
there is no declination of the magnetic needle. They 
are connected by an imaginary line called a line of 
no declination, or agonic line, which passes round the 
earth, nearly from north to south, cutting the Equator 
at right angles. Besides the agonic line there are a 
number of other imaginary lines called isotonic lines. 
Each one of these connects places on the earth's 
surface where the declination is the same. A map 
drawn out representing the isogonic lines is called 
a declination map, 2 and is of great service to 

1 The daily variations follow the course of the sun, or rather 
seem to make an effort to do so, for they are very slight. 

2 That is to say, it is so-called by scientists. Sailors know it as 
map. 



130 MAGNETISM part ii 

mariners, whose compass is in like manner called a 
declination compass. It is not known who was the 
first inventor of this valuable instrument, but it was 
in general use in Europe in the thirteenth century, 
though in a much more primitive form than any 
with which we are familiar. It consisted merely of 
a magnetic needle set floating in a basin of water by 
means of a cork or of two straws, and nothing then 
was understood about " declination," the needle was 
supposed to point due north and south. 1 Hundreds 
of years before the compass was used in Europe, it 
was know r n to the Chinese, who navigated their ships 
by means of-a magnetic needle pointing south, and it 
is even stated by Humboldt that 1000 years B.C. the 
Chinese had magnetic carriages in which to find their 
way across the plains of Tartary. The compass 
consisted of the figure of a man with a movable arm 
pointing to the south. 

Besides setting itself in the magnetic meridian, 
there is another peculiarity to be remarked in a 
freely-suspended magnetic needle, namely, that if 
placed in a horizontal position, its north-seeking 
pole dips downwards in the northern hemisphere, 
and its south-seeking pole in the southern hemi- 
sphere. This fact was first discovered in 1576 

1 Columbus is supposed to have been the first European to dis- 
cover the declination of the magnetic needle. 



chap. II MAGNETISM OF THE EARTH 131 

by a scientific instrument - maker named Norman, 
and he constructed an inclination compass or dip 
needle, designed to show the angle of dip or inclina- 
tion which the magnetic needle makes with the 
horizon. Far more delicate and accurate instruments 
are now in use at Kew and other great observatories, 
where daily and minute records are kept of the 
three magnetic elements, as they are called, namely, 
the intensity of the earth's magnetism at the particular 
spot, and the declination and inclination of the needle. 
Just as there are places where there is no declina- 
tion, so there are places where there is no inclination, 
and these are situated at the farthest possible distance 
from the magnetic poles. The imaginary line which 
connects them, therefore, roughly follows the course 
of the geographical equator, cutting it at two points 
almost exactly opposite to each other, and situated 
one in the Atlantic and one in the Pacific Ocean. This 
line is called the magnetic equator or aclinic line. 
Isoclinic lines are those which connect places on the 
earth's surface where the inclination of the needle is 
the same. At the north magnetic pole the inclination 
needle is vertical, and could the south magnetic pole 
be reached its position would of course be the same 
there. We have therefore very clear evidence that 
the magnetic force affecting the needle does really 
reside in the earth itself and not at any point above 
10 



MAGNETISM 



PART II 



its surface, for were this the ease there would be no 
inclination. Fig. 14 is a magnetic map of the world 




Fig. 14. —Magnetic Map of the Northern Hemisphere. A, North Magnetic 

Pole. 



showing both the isogonic and isoclinic lines of the 
northern hemisphere. It is not possible, however, 
to construct such maps accurately once for all 
like geographical maps. The inclination, like the 
declination of the needle, undergoes annual and other 



chap. II MAGNETISM OF THE EARTH 133 

variations, and in fact the minute and careful ob- 
servations which have been made of late years show 
that the magnetism of the earth is in as continual a 
state of fluctuation as the waters of the ocean. 1 
Nevertheless, it is possible to determine what these 
fluctuations are likely to be for a few years in ad- 
vance, and to construct maps which will hold good 
approximately for that length of time. The close 
study accorded to terrestrial magnetism has brought 
out many curious facts concerning it, one of which is 
a certain periodicity in the appearance of magnetic 
storms which makes their greatest frequency coincide 
with the maximum period of sun-spots, i.e. every ten 
or eleven years. It would seem, in fact, that there 
is some remarkable connection between the sun and 
the earth's magnetism. One specially striking proof 
of this occurred in 1851, when a luminous mass was 
seen to cross a sun-spot, and at the same time the 
magnetic needle at Kew underwent great perturba- 
tions. Subsequent inquiries brought to light the fact 
that at the same moment one of the most violent 
magnetic storms ever known was goin£ on in various 
parts of the earth. 

1 These fluctuations in the magnetism of the earth cause what 
are known as "earth currents," often very troublesome in telegraphy. 
They always occur during magnetic storms, and there are also ex- 
ceedingly weak daily earth currents, flowing from the magnetic poles 
towards the equator. 



134 MAGNETISM part ii 

Before closing this chapter it is necessary to call 
attention to the fact that the effect of the earth's 
magnetic force on the magnetic needle is simply 
directive, causing it always to take up a particular 
position, but not imparting to it any power of loco- 
motion. This is proved by observing the behaviour 
of a floating magnetic needle. It does not move 
towards the north, though it sets itself so as to point 
in that direction. Yet if we were to hold a magnet 
near the edge of the vessel, the needle would in- 
stantly move towards that, which seems to necessitate 
an explanation of its different behaviour with regard 
to the earth. The fact is that in ordinary cases the 
needle moves towards the magnet because the nearer 
pole of the latter acts more strongly on one pole of 
the floating needle than on the other. In the case 
of the earth, however, both its poles are so far away 
from the needle that the one which is nearest exerts 
equal and opposite forces on the latter, forming 
what is called the terrestrial magnetic couple, tending 
to turn the needle round but not to cause any 
movement of translation. 1 

It has already been stated that the earth is able 
to induce magnetism in steel or iron bars, but, in fact, 

1 Any two equal forces acting in opposite parallel directions to 
each other, on a rigid body tend to produce a movement of 
rotation. 



chap, ii MAGNETISM OF THE EARTH 135 

any steel or iron objects or masses of those metals 
are affected in the same way. Fire-irons which have 
been allowed to stand for a considerable time in a 
vertical or inclined position become magnetised, the 
lower end being a north-seeking and the upper end 
a south-seeking pole, 1 and the same remark applies 
to railings, lightning-rods, etc. Objects made of steel 
or of cast iron retain this state of magnetisation, but 
pure, soft iron cannot do this, as it possesses no 
retentivity, or coercive force as it is called. For this 
reason a bar of soft iron magnetised by the earth has 
the magnetism of its poles immediately reversed with 
the reversal of their position if it be tapped, that 
which was a north-seeking becoming a south-seeking 
pole if turned upwards. This phenomenon is very 
rarely observed, however, in common objects, as the 
ordinary iron of commerce is not perfectly pure, and 
therefore possesses a slight retentivity, even the tools 
in a smith's shop showing faint signs of magnetisation. 
The most important effects produced in this way 
by the earth's magnetism are those on iron ships, 
which during the process of building become, owing 
to the hammering they receive while under the 
influence of the earth's magnetism, permanently 
magnetised, and consequently able to exert a disturb- 

1 That is, in the northern hemisphere. In the southern the 
reverse would be the case. 



136 MAGNETISM part ii 

ing influence on the compass-needle, which is thus in 
many positions of the ship unable to lie in the true 
magnetic meridian. Such a result is of course disas- 
trous to navigation, and various methods of obviating it 
are resorted to. The use of compensating magnets, i.e. 
masses of iron placed in such a position with respect to 
the compass that they neutralise the effect of the ship's 
magnetism on it, is one ; but here a difficulty arises 
from the fact that after a first voyage the magnetism 
of the ship generally alters, becoming less strong than 
it previously was owing to the buffeting of the waves. 
In fact, for a considerable period every voyage makes 
a difference to the ship in this way, and the compen- 
sating magnets have to be frequently altered lest 
they in their turn should disturb the compass-needle 
by over-compensating the magnetism of the vessel. 
Fortunately, after a time this does become really 
fixed, but until then so great are the difficulties 
attending the use of compensating magnets that they 
are frequently dispensed with, and a table of errors 
drawn up by careful observation of the magnetism of 
the ship and continual comparison with the indica- 
tions of the compass-needle is trusted to instead. In 
the Eoyal Navy both methods are employed. The 
extreme importance of care in this respect is demon- 
strated by the fact, that the loss of ships has been 
known to occur owing to errors in the compensating 



chap. II MAGNETISM OF THE EARTH 137 



magnets which rendered the compass directions un- 
true. Such a disaster has, however, never occurred 
in the Eoyal Navy, — a fact on which the authorities 
justly pride themselves. 

Frequently a standard compass is placed in the 
masts, so that it may be as far removed as possible 
from the influence of the ship's magnetism. 



PART III 
CURRENT ELECTRICITY 

CHAPTEE I 

THE GALVANIC BATTERY 

Definition of an electric current — Direction of the current — Gal- 
vanic battery — Description of simple voltaic or galvanic cell — ■ 
Effects of the current — Its cause— Poles — Electro-motive force 
■ — Resistance — Ohm's law — Difference between electro-motive 
force of cells in series and in parallel — Weakening of current 
through polarisation — Daniell's cell — Grove's and Bun sen's 
batteries — Principle common to all batteries — Secondary- 
batteries — Possibility of obtaining them due to polarisation — - 
Self- induction — " Extra - current " effects — Space surround- 
ing wire conveys the electric current as well as the wire 
itself — Nature and effects of an electric current the same from 
whatever source it is supplied — Thermo-electric currents and 
the thermopile. 

It can hardly be said that in the section devoted to 
static electricity no mention was made of currents, 
for all discharge was shown to be a flow, and flow 
is but another word for current. That which takes 



chap. I THE GAL VANIC BA TTER Y 1 39 

place in discharge, however, is momentary, and by a 
current of electricity a continuous flow is nearly 
always understood. This continuous flow is caused 
by a maintained difference of potential between one 
point and another, and when once set up it does not 
cease until the potential is equalised, any more than 
a river would cease flowing unless its whole bed 
became level, or its springs were dried up. 

By the direction of an electric current is in- 
variably meant the floiv from positive to negative, 
though it must not be forgotten that there is always 
a negative current as well, whose course is exactly 
opposite, i.e. from negative to positive. In practice 
this is usually entirely ignored, a fact which does not 
render it one whit the less important and interesting 
to those whose inquiries turn towards the nature of 
electricity. ISTo satisfactory theory can be propounded 
which does not take into account the double current. 

One of the easiest and most familiar ways of pro- 
ducing an electric current is by means of the Voltaic 
or Galvanic hattery, discovered towards the close of 
the last century by the researches of two eminent 
Italian scientists, Volta and Galvani, working inde- 
pendently of each other and on some theoretical 
points diametrically opposed. 

A battery consists of a larger or smaller number 
of cells or " elements " exactly like each other, and 



140 



CURRENT ELECTRICITY 



PART III 



the following is a description of the earliest form of 
cell. A strip of copper and a strip of zinc are placed, 
not touching each other, in a glass or porcelain vessel 
containing an acid liquid (a very weak solution of 
sulphuric acid is generally used), and are connected 
by copper wires, starting one from the zinc and the 
other from the copper p ate (Fig. 15). When these 

wires are joined a current 
of electricity is set up which 
flows from zinc to copper 
through the liquid, and from 
copper to zinc through the 
wires, that is right round 
the circuit, and it continues 
to flow thus till the wires are 
disconnected. The junction 



c: 



c^r^ 



-+ 



^JN. 



Fig. 15.— single Galvanic Cell, in- between the wire and the 

dicating the direction of the 

current through the liquid and COpper plate is Called the 
wires. Z, zinc plate ; C, copper . . , 7 , 

plate. The terminal wires join positive pole Or electrode, 

Z to the copper plate of an ad- ^ « t * 4-W p fW f/h P 

joining ceil, and c to the zinc because it is mere inau ine 
plate of an adjoining ceil. positive current through the 

wires begins, and the junction between the wire and the 
zinc plate is called the negative pole or electrode. This 
current produces all the same effects as a discharge. 
The magnetic needle is deflected by it, chemical de- 
composition is caused, a thin wire becomes heated, and 
if the ends of the two wires are placed on the tongue 






chap. I THE GALVANIC BATTERY 141 

a peculiar taste is noticed. All these effects are 
greatly enhanced by placing a number of cells in 
series, connecting the zinc plate of one to the 
copper of the next, and so on, the wires being 
fixed to the terminal zinc plate at one end and the 
terminal copper plate at the other. When the cells 
are thus connected, the current flows from one to the 
other, but they may be arranged in parallel, i.e. all 
the zinc plates connected to each other, and all the 
copper plates to each other. The current then 
divides itself between the cells. 

At this point the question naturally arises, What 
causes the current ? A long and fierce war, perhaps 
hardly yet terminated, was waged by the disciples of 
Volta and Galvani respecting the right answer to be 
given. The former maintained that the current was 
due to contact of dissimilar metals, the latter to 
chemical action, and since both these causes exist in 
the battery, and both produce difference of potential, 
which is as necessary to the existence of a current 
as difference of level is to a flow of water, it seems 
very difficult to decide between the two. Probably 
the right way of addressing the disputants would 
have been in the words of the traveller called upon 
to arbitrate in the far-famed quarrel concerning the 
colour of the chameleon, 

" You both are right and both are wrong." 



142 CURRENT ELECTRICITY part hi 

It probably is the contact of dissimilar metals which 
causes the difference of potential in the first instance, 
and the effort to maintain this potential difference 
requires chemical action, so that the chemical action, 
to be presently described, maintains the continuous 
flow. 

The work done in a galvanic battery may be com- 
pared to the work done by a pump. The pump 
raises water from a low to a high level, in opposition 
to the natural tendency of water to flow from a high 
to a low level. Work is thus expended on the water 
which is reproduced, minus the amount wasted in 
friction in the pump, by the water as it flows back to 
its original level. In a galvanic battery electricity 
is raised from a low to a high potential, in opposition 
to its apparent tendency to flow from a high to a 
low potential. Work is thus done on electricity 
which is reproduced, minus the amount wasted in 
overcoming the resistance of the battery cells, by 
electricity as it flows back through the outer circuit 
to its original potential. What happens inside the 
cells of a battery, then, is that an electric current is 
driven against a difference of potential (or of electric 
level), and that a difference of potential is con- 
sequently produced and maintained between the 
terminals or electrodes of the battery. What 
happens outside the battery is that a flow of elec- 



chap. I THE GALVANIC BATTERY 143 

tricity takes place between the terminal at high 
potential to that at low potential, so that the direc- 
tion of the current is with the slope of potential, and 
work is done by it on its road. Electricity is raised 
from a low to a high potential inside the battery, and 
caused to flow from a high to a low potential in the same 
circuit outside the battery, by what is called electro- 
motive force. No electric current can exist anywhere 
without an electro-motive force, and since in very 
many instances it may also be said that no current 
can exist without a difference of potential, these two 
terms, electro-motive force and difference of potential, 
are often regarded as interchangeable. Yet they do 
not express the same thing. Electro -motive force 
may cause, or may be the result of, difference of 
potential, or may exist without it, whereas difference 
of potential cannot exist without electro -motive 
force. Moreover, it is necessary to remember that 
though electro-motive force is thus named, it is not, 
accurately speaking, a force at all. It does not act 
on matter, which is the characteristic of force. It 
acts on electricity, whatever that may exactly mean. 
And perhaps the most comprehensive definition 
which can be given of it, is that it is "the ratio 
of the rate of doing work in the circuit to the 
current flowing." 

It has already been stated that, however good 



144 CURRENT ELECTRICITY part hi 

the conductor through which an electric current is 
flowing may be, the latter always encounters a certain 
amount of obstruction or resistance, on its road, which 
may be considered analogous to friction in the case of 
ordinary matter. The power of the current to over- 
come this resistance, depends on the force with which 
it is being driven along (on the electro - motive 
force), and the strength of the current, by which is 
meant the quantity of electricity flowing per second 
past a cross-section of the conductor conveying the 
current, increases in direct proportion to the increase 
of potential difference. It was ascertained first 
by Ohm, and has since been carefully proved 
by others, that in a metallic conductor of the same 
material, dimensions, and temperature, the ratio 
of potential difference to current - strength never 
changes, and may be called the resistance of that 
particular conductor. This statement is known as 
Ohm's law, and has now been proved true for liquids. 
Its various applications are of the greatest importance 
and interest to practical electricians. 

When the cells of a battery are connected in 
series, the sum of their respective electro -motive 
forces is the electro-motive force of the whole battery. 
When they are in parallel, the electro-motive force of 
one cell equals that of the battery. The electro- 
motive force of any cell is independent of its size, 



CHAP. I 



THE GAL VANIC BA TTER Y 



145 



and is affected only by the materials of which it 
is made. 

The current in the earlier batteries could only be 
maintained a short time, owing to the rapid decrease 
in its strength caused by the very chemical action 
which produced and sustained it. This chemical 
action consists in the dissolving of the zinc in the 
acid, by which means 



^^r\ 



sulphate of zinc is 
formed, and hydrogen 
gas set free. The 
latter forms in bubbles 
on the copper plate, 
and there does a two- 
fold mischief. In the 
first place, being a bad 
conductor, it greatly 
diminishes the effect- 
iveness of the copper ; 
in the second, being 
itself an electro-posi- 
tive substance (becom- 
ing positively electri- 
fied by contact with 
other substances), it 
tends to set up an opposing electro-motive force in 
the battery ; in other words, a second positive 




Fig. 16.— Daniell's Cell. Z, zinc plate; 
C, copper plate ; E, porous earthenware 
partition. The vessel containing the 
liquid is represented as made of glass, 
but glazed stoneware is very frequently 
used, and a zinc rod often takes the 
place of C in the figure, the copper plate 
being then situated at Z. 



146 CURRENT ELECTRICITY part ii 

current flowing in an opposite direction to the 
first, and consequently much weakening its action. 
A battery in this condition is said to be polarised, and 
various cells have been made by different electricians 
to prevent its occurring, and so render the current 
constant, i.e. enable it to maintain the same strength 
for along time together. Daniell's (Fig. 16) was the 
first, and both it and various modifications of it are 
still much used. Instead of one, two liquids are em- 
ployed in it, one in contact with the zinc, and one with 
the copper plate, which are sometimes rolled cylinder 
fashion, and divided by a porous, unglazed earthen- 
ware partition. The liquid which is in contact with 
the copper plate contains sulphate of copper as well 
as sulphuric acid. The effect of the whole arrange- 
ment is to intercept the hydrogen on its road, and 
instead of it, particles of copper drawn from the 
sulphate of copper contained in the liquid are de- 
posited on the copper plate, which consequently 
cannot lose in efficiency. 

Another form of battery much used is Grove's, 
where platinum is substituted for copper. Though 
capable of maintaining a current of the same strength 
for several hours at a stretch, the expense of the 
platinum constitutes a disadvantage in this battery ; 
and Bunsen contrived one greatly used in laboratory 
experiments in which graphite, hard gas carbon 



chap. I THE GALVANIC BATTERY 147 

obtained from the interior of gas - retorts, is used 
instead of either platinum or copper. 

There are many other batteries unnecessary to 
describe here, their suitability varying with the pur- 
pose for which they are employed. One principle, 
however, is common to all. No battery can produce 
a current giving sensible effects unless there is a 
sensible consumption of its materials by chemical 
action, any more than a fire will give out sensible 
heat without a sensible consumption of coal. The 
material which is " burned " in a battery is generally 
zinc, which being almost at the head of the electro- 
positive series (see p. 39) and very readily oxidisable, 1 
has not hitherto been replaced by another less expensive 
substance. In consequence of this it was found im- 
possible to utilise electric currents on a large scale 
in any work, such as electric lighting, where power- 
ful currents were required, till some other method 
than that of galvanic batteries could be employed 
to generate them, for the consumption of material 
being proportional to the strength of the current, 
a large quantity of zinc must be used up in order 
to produce a powerful current for a considerable time. 
In telegraphy, where a weak current suffices, this 

1 It is absolutely necessary that one of the metals employed in 
a battery should have a great affinity for oxygen, as it is this 
affinity which first starts the chemical action in the cells. 
11 



148 CURRENT ELECTRICITY part hi 

objection does not apply. It should be mentioned 
that it is necessary to use either perfectly pure or 
amalgamated zinc, otherwise chemical action goes 
on, and the zinc dissolves even when the current 
is not passing, which " local action/' as it is called, 
causes much needless waste. 

Before quitting this subject, mention must be 
made of what are called secondary batteries, in 
which the energy of a current may be stored up as 
chemical work, and again given out in the form of 
electric energy. They are also called storage latteries, 
and an opinion is often held by the unscientific 
that electricity itself is stored. Such is not the 
case, however ; it is the energy of the current which 
is stored in the form of the products of chemical 
decomposition, and when this stored-up energy is 
freed, an electric current is again set up and chemical 
recombination begins. A secondary battery cannot 
commence to w r ork of itself. It needs in the first 
instance to be "charged," i.e. a current must be 
passed through its cells from an external source, in 
order to produce the chemical decomposition in which 
the work of storage consists. When this has been done 
for a sufficient length of time, the two batteries are 
separated, and the poles of the secondary being con- 
nected, a current is immediately set up, having all 
the properties of, and being able to perform the same 



chap. I THE GALVANIC BATTERY 149 

work as that from an ordinary galvanic battery. 
There is no necessity to use the secondary battery 
at once ; it will remain " charged " for a considerable 
time, and in this fact consists its importance and 
convenience. The method of charging by a galvanic 
battery, however, is very expensive, and in conse- 
quence no wdde use could be made of the storage 
principle until other means of charging were avail- 
able. The dynamo machines, to be described in 
a future chapter supplied this want, and secondary 
batteries have consequently come into great request, 
being specially useful in electric lighting and 
locomotion. 

The possibility of obtaining secondary batteries 
is really due to polarisation. We have already seen 
(p. 145) how an opposing electro-motive force may be 
set up in an ordinary battery by the deposit of 
hydrogen on the negative electrode, causing after a 
time a second positive current flowing in an exactly 
opposite direction to the first, and so weakening its 
action. It is this, the polarisation current, which is 
utilised in secondary batteries, and consequently the 
current in them flows always in the opposite direction 
to that of the charging battery. Ritter first dis- 
covered the principle of secondary batteries, called 
also accumulators, in 1803, and many years after 
the eminent French electrician, Gaston Plante, showed 



150 CURRENT ELECTRICITY part hi 

how it could be turned to practical use. Faure's 
storage battery, an improvement on Plante's, is now 
most generally used. 

It is usual and convenient to speak of the con- 
ducting wires as though they alone conveyed the 
current, but theoretically, and as a true explanation 
of what happens this is not the case. We have 
already seen what an important part is played in the 
phenomena due to static electricity by the insulating 
medium, and its function in the case of electric 
currents is equally necessary, for it will easily be 
understood that though the wires appear to convey 
the current, the surrounding space must take part in 
the action also, because within such a space the 
magnetic needle is affected, and other magnetic and 
electric phenomena occur. We cannot therefore 
regard the wire so much as a sort of pipe through 
which something or other is passing, as the centre 
of a disturbance in the ethereal medium, which 
disturbance is propagated along the outside as well 
as through the wire. It is, in fact, now considered 
that the energy of an electric current travels entirely 
through the insulating medium, and not through the 
wire at all, the function of the latter being to dis- 
sipate, not to transmit, the energy it receives. By 
this dissipation, however, it enables the surrounding 
medium to continue transmitting more energy, instead 



chap. I THE GAL VANIC BA TTER Y 151 

of taking up a passive strained condition, such as 
exists, for instance, in the dielectric layer of a 
condenser. 

This fact, that it is really the space round the 
wire, and not the wire itself, which conveys the 
energy of an electric current, explains a phenome- 
non which was for some time not understood, 
viz. that an electric current does not instantan- 
eously rise to its full strength when circuit is 
made, nor instantaneously cease when it is broken. 
A very slight, but still measurable, delay occurs 
in both instances, and in the latter the sudden 
breaking of the circuit will often occasion sparks, 
showing that the current, unable to stop at once, 
bursts through the insulating medium interposed 
with an outbreak of heat and light. Water, which 
has been several times used as presenting an una- 
logy to electrical phenomena, affords one also here. 
Water enclosed in a pipe cannot be set in motion 
suddenly, or if already in motion cannot be suddenly 
stopped, except by the exertion of a force which is 
very likely to burst the pipe. With water these two 
effects are due to inertia, a universal property of 
matter which can neither start nor stop moving 
unless force be brought to bear on it. Since an 
electric current exhibits the same peculiarity, we are 
naturally led to ask whether that also possesses inertia, 



152 CURRENT ELECTRICITY part hi 

and the interest of the question lies in the fact that 
if it did, electricity would be proved to be a form of 
matter, however widely different that form might be 
from those with which we are familiar. But though 
in the instances cited above (which used formerly to 
be called "extra-current" effects), electricity appears 
to possess inertia, in other equally important ways 
it seems entirely devoid of it. Inertia where it really 
exists produces well-defined mechanical effects, and 
examined by any mechanical means an electric 
current shows no sign of it. The fact is that the 
effects observed on making and breaking circuit, as 
well as others of a similar nature, are due not to the 
inertia of electricity, but to the electro -magnetic 
inertia of space (or rather of the medium which fills 
space), and this is quite a different thing. As we 
have seen, the space surrounding a wire conveying 
an electric current acquires the property of producing 
magnetic effects. Such a space must therefore be in 
a state of magnetisation ; but it neither acquires nor 
loses this condition instantaneously, and in conse- 
quence causes those phenomena (known as self- 
induction phenomena,) which appear to be due to the 
inertia of the current itself. 

In whatever way an electric current is given rise 
to, its nature and effects are essentially the same ; 
and therefore though one source only, the galvanic 



chap. I THE GALVANIC BATTERY 153 

battery, has yet been described, it will be well to 
give in the ensuing chapters some more detailed 
account of the various effects produced. Before 
doing so, however, a brief mention may be made of 
what are called tliermo - electric currents. These 
arise from setting up a difference of temperature 
between two junctions formed of two different 
metals, the effect being more marked when bismuth 
and antimony are used than with any other metals. 
Two metals joined for the purpose of giving rise to 
an electric current through inequality of temperature 
are called a pair, and a number of these pairs may 
be united so as to form a kind of battery, which is 
known by the name of a thermopile, every alternate 
junction being either heated or cooled above or 
below the temperature of the rest of the circuit. 
When a difference of temperature is set up between 
two junctions of bismuth and antimony the current 
flows from bismuth to antimony across the hotter 
junction, and from antimony to bismuth across the 
colder, the hotter junction being cooled and the 
colder warmed during the process, so as to bring 
them to the same temperature as the rest of the 
circuit, when the electro-motive force (called thermo- 
eleetro - motive force), and consequent difference of 
potential, causing the current cease. The currents 
thus produced are of a low electro -motive force, 



154 CURRENT ELECTRICITY part ill 

though some thermopiles have been constructed 
which generate currents strong enough to depose 
metals from their solutions, and they have even 
been made of some practical use in this way. The 
most usual and important function of the thermo- 
pile, however, is to act as an extremely delicate 
thermometer able to indicate the very smallest 
changes in temperature, and for this purpose it is 
invaluable. 






CHAPTEE II 

CHEMICAL AND PHYSIOLOGICAL EFFECTS OF THE 

CUKRENT 

Difference In the way solid and liquid conductors convey an electric 
current — Analogy with heat — Electrolytes — Electrolysis — 
Electrodes — Anions and kathions — Deposition of metals by 
electrolysis — Electrolysis of water — The voltameter — Free 
atoms only appear at the electrodes — Grothuss' hypothesis — 
Physiological effects of the current — Galvani's experiments — 
Results of recent experiments — Exciting effect of extraneous 
currents on living nerves — Difference between the physiological 
effect of the passage of a galvanic current and a Leyden jar 
discharge — And of continuous and alternating currents. 

Chemical Effects. 

An electric current does not flow in the same way 
through solid conductors and through liquids. In 
the former it does not travel with the molecules of 
matter, but in some way through them, whether we 
picture it doing so as water filters through sand, or 
as passing from one molecule to another like heat. 
Heat itself, however, travels in two ways, by what 
we call conduction in solids, and by convection in 



156 CURRENT ELECTRICITY pabt hi 

liquids and gases. In conduction there is an 
increased vibration of the molecules communicated 
from one to the other ; in convection there is an 
actual double journey of molecules, the hot light 
ones rising to the top, and the cold heavy ones 
sinking to the bottom. Through most liquid 
chemical compounds electricity also travels by a 
kind of convection. There is a double procession 
of charged atoms, the positive all going one way 
and the negative the other way, and thus the two 
kinds of electricity travel with the particles of 
matter, just as heat does when any liquid is rising 
in temperature. 1 

In order to produce this double procession of 
atoms, however, chemical decomposition must take 
place, and all liquids do not undergo this when an 
electric current is passed through them. It only 
occurs when the liquid is a conductor, and turpentine 
as well as most oils are non-conductors. Again, there 
are liquids which conduct without being decomposed 
by the process. Mercury and all molten metals belong 
to this class, but impure water, as well as acid and 
saline solutions, undergo decomposition whenever a 
current is passed through them, whether inside or 
outside the cells of a battery, and they are known 

1 See Modern Views of Electricity, by Dr. Oliver Lodge, F.K.S., 
p. 66. 



ch. II CHEMICAL AND PHYSIOLOGICAL EFFECTS 157 

in consequence as electrolytes. The process of de- 
composition is called electrolysis, a name originally 
bestowed by Faraday, and abbreviated from electro 
analysis. The vessel containing a liquid undergoing 
electrolysis is called an electrolytic cell, and the ends 
of the wires leading from and to the battery, or the 
strips of metal (usually platinum) to which the wires 
are connected, and which dip into the liquid, are the 
electrodes. The positive electrode is called the anode 
and the negative the kathode. 

The atoms set free by decomposition and attracted 
to the respective electrodes have been already men- 
tioned. Faraday gave them the name of ions, those 
which appear at the anode being anions (the ones 
which go up), and those which appear at the kathode 
kathions (the ones which go down). The latter are 
regarded as being electro-positive because they move 
with the positive current towards the negative elec- 
trode, and the former as electro-negative because they 
move against the positive current towards the posi- 
tive electrode. All metallic atoms are kathions — 
that is, they appear at the negative electrode, and 
several metals have been discovered through elec- 
trolysis, by being disengaged from the substance 
with which they were united, and deposited by them- 
selves at the kathode. Potassium is one of these, 
and was discovered by Sir Humphry Davy. 



158 



CURRENT ELECTRICITY 



PART III 



When very pure water is submitted to electrolysis, 
there must be added to it a few drops of sulphuric 
acid, perfectly pure water appearing to act as a non- 
conductor. This being done, however, the process of 
decomposition commences at once, oxygen being 
evolved at the anode, thereby proving itself to be 
electro-negative, and hydrogen at the kathode, thus 
showing that it is electro-positive. Near]y twice as 
r\\i much hydrogen is 

given off as oxygen in 
consequence of the 
chemical composition 
of water, which con- 
sists of two parts of 
hydrogen to one of 
oxygen. If it is de- 
sired to collect the 
gases thus set free, an 
apparatus like that 
shown in Fig. 17, and 
known as a voltameter, 
must be used. It con- 
sists of a vessel containing slightly acidulated water, in 
which are immersed two strips of platinum connected 
by wires with the respective poles of a battery. The 
strips of platinum are the electrodes of the voltameter, 
and platinum is used because it resists the action of 




Fig. 17.— Voltameter. A, anode ; 
K, kathode. 



ch. ii CHEMICAL AND PHYSIOLOGICAL EFFECTS 159 

every acid, and is not easily oxidisable. Consequently 
it does not tend to set up other chemical actions besides 
that of the current in the voltameter. The two in- 
verted tubes over the platinum strips serve to collect 
the gases ; bubbles appear at the surface of the water 
with which they are originally filled, and as this, 
happens the level of the water sinks, the upper 
part of the tube over the anode becoming filled with 
oxygen, and that over the kathode with hydrogen. 1 
The voltameter affords a very direct way of measuring 
the strength of an electric current, because the amount 
of chemical action which takes place in a given time 
is directly proportional to the strength of the current, 
and within wide limits no other consideration need 
be taken into account. Copper voltameters are 
frequently used for practical purposes. Two plates 
of copper are immersed in a solution of copper sul- 
phate (blue vitriol), and serve as the electrodes, that 
which is to be the kathode having been first carefully 
weighed. When the current is passed through the 
cell, particles of copper are drawn from the solution 
and deposited on the kathode, while the anode grad- 
ually dissolves in the exact proportion necessary to 
replace the copper taken from the solution. After a 

1 The form of voltameter above described, though still frequently 
seen, is becoming very antiquated, and other better forms have 
been devised. 



160 CURRENT ELECTRICITY part ill 

given time, the kathode is removed and again weighed, 
its increase in weight indicating precisely the amount 
of current that has passed. Though this method of 
measurement is exceedingly accurate and direct, it can 
only be adopted in the case of large currents (such as 
those used in electric lighting), because in the case 
of small currents, though the amount of chemical 
action taking place in a given time is always directly 
proportional to the current strength, it is too minute 
to be appreciable for many hours, or perhaps days. 
Weak currents are therefore measured by their 
magnetic, not their chemical effects, as will be 
presently described. 

There is one very curious fact regarding electro- 
lysis which must not be left unmentioned, viz. that 
the separated atoms never make their appearance 
except at the electrodes, and however many cells 
they may have to pass through before arriving at 
their respective destinations, nothing whatever is seen 
of them on the road. The only explanation which 
seems to account satisfactorily for this remarkable 
phenomenon is that known as Grothuss' hypothesis. 1 

1 Grothuss' hypothesis has been modified in order to meet the 
further development of chemical science, and it is now more gener- 
ally supposed that the molecules and atoms of a liquid being 
always in motion, the passage of an electric current through them 
controls the direction of that motion, by causing the electro-posi- 
tive atoms to move towards the kathode, and the electro-negative 



ch. ii CHEMICAL AND PHYSIOLOGICAL EFFECTS 161 

He supposed that each molecule of the electrolyte 
underwent a continual decomposition and recomposi- 
tion. Thus, taking water as an instance, each mole- 
cule of which is composed of one atom of oxygen to 
two of hydrogen, the first molecule decomposed at 
the positive electrode, sets free one atom of oxygen 
and two of hydrogen. The latter immediately com- 
bine with the oxygen of the second molecule, whose 
hydrogen is in turn set free, and passes on to combine 
with the oxygen of the third molecule, which is 
decomposed and recomposed in like manner, and thus 
the process continues till the negative electrode is 
reached, where the last two atoms of hydrogen, having 
no oxygen to combine with, appear free. 

Physiological Effects, 

Galvani was the first to draw the attention of the 
scientific world to these, and he himself was accident- 
ally attracted to the subject by observing one day that 
the legs of some newly-killed frogs underwent violent 
contractions at every discharge of an electrical 
machine with which he was experimenting. This 
effect was due to the " return shock," viz. to the 
frogs' legs having become charged by induction owing 
to the near neighbourhood of the electrical machine, 

towards the anode, thus causing the decomposition of the liquid 
and the appearance of the free atoms. 



162 CURRENT ELECTRICITY part in 

and consequently discharging themselves when it 
did. Not long afterwards Galvani discovered that if 
a living nerve and muscle are touched by two dis- 
similar metals in contact, an electric current is set up 
and the muscle contracts. Subsequently he proved 
that a single metal would have the same effect, and 
still later that metal could be dispensed with alto- 
gether, and the contraction produced by touching the 
nerve at two different points with a muscle taken 
from a living frog. Since his time these experiments 
have been tried on other animals warm-blooded r as 
well as cold-blooded, and their scope greatly extended. 
From these researches it has been ascertained, first, 
that the power of contracting on the passage of an 
electric current is a distinguishing property of pro- 
toplasm, the physical basis of all animal and vegetable 
life ; secondly, that not only do extraneous currents 
produce certain defined physiological effects, but also 
that electric currents exist in the living nerves and 
muscles of all animals, independent of any external 
stimulus, and that they cease with death, thus estab- 
lishing an intimate connection between electricitv 
and vital phenomena. What this connection really 
is, however, remains unknown, and in any case elec- 

1 The experiments are much more difficult to carry out in the 
case of warm-blooded animals, because their muscles do not retain 
vitality so long after the general death of the system. Neverthe- 
less the same results have been obtained. 



CH. II CHEMICAL AND PHYSIOLOGICAL EFFECTS 163 

tricity and life are not, as some people seem to 
suppose, synonymous. 

The effect of extraneous currents on living nerves 
is invariably to excite them to action. Thus, if a 
feeble current be passed through the eyeball, a 
brilliant flash of light is seen, owing to the stimulus 
given to the optic nerve. If the ear be treated in 
the same way, musical sounds are heard. A current 
passed through the tongue causes a peculiar taste, 
and applied to the ordinary nerves of sensation a 
pricking and stinging are produced. These effects 
are mostly momentary, occurring only when the circuit 
is made or broken, but if this be done frequently and 
rapidly, an equally frequent and rapid succession of 
the effects may be produced. Where the current is 
strong enough to cause contraction, tetanus may 
ensue if the current be interrupted at frequent and 
rapidly-recurring intervals, owing to one contraction 
not having time to pass off before another commences. 
The same effect can be produced by alternating 
currents {i.e. currents flowing alternately in opposite 
directions). 

A galvanic current does not usually give a shock 
like a Leyden jar, but it will do so, when circuit is 
made or broken, if the number of cells in the battery 
employed, is sufficient to give rise to a high electro- 
motive force, for it is the difference in this respect 
12 



164 CURRENT ELECTRICITY part in 

between a battery and a Ley den jar which causes 
the difference in their physiological effects. The 
battery gives out larger quantities of electricity than 
the Leyden jar, but the difference of potential between 
its poles is far less than that between the two coat- 
ings of the jar, and consequently the electro-motive 
force of the latter is much the greater. The Leyden 
jar discharge is like a small stream of water falling 
from a great height ; the battery current like a large 
stream flowing over a very gently-inclined bed, and 
it will be easily understood how much more likely a 
" shock " is to occur in the former than in the latter 
case, especially as the resistance of the human body 
is very high and requires considerable electro-motive 
force to overcome it. 

The physiological effects produced by continuous 
and alternating currents are also different, and the 
latter are both more painful and more dangerous 
than the former. A person accidentally touching a 
wire, conveying a continuous current of an electro- 
motive force not high enougli to give a shock causing 
unconsciousness, could release himself at will. In 
the case of an alternating current he could not do so, 
and would suffer painful muscular contractions while 
remaining fixed. Moreover, the human body can bear 
without danger, continuous currents of a much higher 
electro-motive force than it can alternating currents. 



CHAPTEE III 



MAGNETIC EFFECTS OF THE CURRENT 

First discovery of the deflection of the magnetic needle by the 
electric current — (Ersted's experiments — Ampere's memoria 
tcchnica — Use of galvanoscope — Of galvanometers — Long and 
short coil instruments — Astatic galvanometer — Disturbing 
effect of the earth's magnetism — Compensating magnets — 
Thomson's mirror galvanometer — Relationship between electric 
currents and magnetism — Wire conveying current acts as a 
magnet — Possesses a magnetic field — Magnetic behaviour of a 
single wire loop — Equivalent to magnetic shell of the same 
dimensions — Application of Ampere's rule — Experiment with 
metallic ring attached to floating battery — Modern theory of 
magnetism — Amperian currents — Production of rotation 
characteristic of magnetism — Tendency of a single magnet 
pole and an electric current to revolve round each other — 
Rotation of liquid conductors under magnetic influence. 

The magnetic effects of the current are of the greatest 
importance, and so long ago as 1803 it was known 
that an electric current deflects the magnetic needle 
from its true position, tending to place it at right 
angles with the conducting wire, so as to make it 
lie in fact across the current. No use was made of 



1 66 



CURRENT ELECTRICITY 



PART III 



this discovery, however, nor was it even published, 
and to (Ersted of Copenhagen belongs the honour of 
having established the fact by careful experiments 
and brought it to the notice of the scientific world. 
Tig. 18 enables (Ersted's experiments to be under- 




Fig. IS. — W W, wires fixed in the magnetic meridian ; M M, magnetic needle 
lying in its normal position ; M' M', the same deflected by a current 
passing from south to north above it, as indicated by the arrow. 

stood. A magnetic needle is placed between two 
wires lying in the magnetic meridian. One is above 
and one below the needle, and both are able to be 
connected with a battery. If a current is made to 
pass from north to south through the upper wire, i.e. 
above the needle, the north-seeking pole of the needle 
is immediately deflected to the east. If it passes 
from south to north through the same wire, as in the 
figure, the deflection of the north-seeking pole is to the 
west. These deflections are exactly reversed if the 
current passes through the wire below the needle. 
The north-seeking pole is then deflected to the east 



chap, in MAGNETIC EFFECTS OF CURRENT 167 

when the current flows from south to north, and to 
the west when it flows from north to south. Ampere 
has given a very curious memoria technica to facilitate 
the remembrance of the various deflections. Suppose 
a man swimming in the conducting wire with the 
current and always turning his face towards the 
needle, 1 all four deflections will then take place 
towards his left hand, so that keeping this rule in 
mind, the following principle will be understood: 
" In the directive influence of currents on magnets 
the north-seeking pole is always deflected to the left 
of the current." 

The stronger is the current, the nearer does the 
deflection of the needle approach to a complete right 
angle with the conducting wire, but it never entirely 
reaches this, the directive action of the current being 
opposed to that of the earth, which tends to keep the 
needle in the magnetic meridian, so that the position 
of the needle must always depend on the relative 
strength of these two forces. 

The deflections of the magnetic needle afford a 
means of indicating the direction and strength of an 
electric current. An instrument constructed for the 
former of these two purposes is called a galvanoscope. 
The simplest of all is made by bending the conducting 

1 To do this when the current passes below the needle he would 
of course have to be lying on his back. 



168 CURRENT ELECTRICITY part hi 

wire into a rectangular form, so that the current passes 
in one direction below the needle and in the opposite 
direction above it, thus acting on it with a double 
strength, because, as we have already seen, a current 
from north to south below the needle deflects the 
north-seeking pole in the same direction as a current 
from south to north above it. 

This apparatus even when improved by having a 
great many turns of wire round the needle instead of 
one, so as to increase the effect of the current, 1 cannot, 
however, do more than roughly indicate whether it 
is strong or weak, or which of two currents is stronger. 
It cannot correctly measure the strength of a current 
relatively to other currents. Yet to know this is of 
the utmost importance in practical work, and for 
this purpose, therefore, galvanometers are employed. 
It is unnecessary to enter into any detailed explana- 
tion of them. The same instrument will not suit all 
purposes, but every galvanometer must have a mag- 
netic needle surrounded by a coil of carefully- 
insulated wire. In long coil instruments the wire is 
turned many, often thousands of times, and is very 

1 Up to a certain limit, the magnetising effect of the current is 
increased with every extra turn of the wire, but since resistance 
is also proportionately increased (owing to the greater length of 
wire which the current has to traverse), it may at length become 
so high as completely to counteract the strengthening effect of 
the coils. 



chap, in MAGNETIC EFFECTS OF CURRENT 169 

fine and thin. These instruments are extremely 
sensitive and are specially suited for very delicate 
experiments, and to include in circuits where the 
resistance is already great. In short coil instruments 
the wire is thicker and has comparatively but few 
turns, and these should be used in circuits of low 
resistance. Some definite controlling force is needful 
in every galvanometer. It may be that of the earth, 
or of some fixed permanent magnet. In this case 
the magnet has to be placed at a considerable 
distance from the needle, so that the latter may be 
in a field of practically uniform strength. This 
condition is always perfectly fulfilled wdiere the 
controlling force is that of the earth's magnetism ; 
but in the case of very sensitive galvanometers, 
where it is necessary that the controlling force should 
be very weak, means often have to be employed to 
obviate the effect of the earth's magnetism. To 
fulfil this purpose in some galvanometers, use is made 
of an astatic 'pair of needles, i.e. two needles of equal 
magnetic strength and size poised carefully one over 
the other in reversed positions, so that the opposite 
poles are confronted, each needle being surrounded 
by a separate coil of wire, the current through one 
coil being sent in the opposite direction to that 
through the other. The result of this arrangement 
is to neutralise the effect of the earth's magnetism 



170 



CURRENT ELECTRICITY 



PAET III 



on the needles through, their mutual re-action, and a 
very high degree of sensibility can thus be attained. 
An astatic galvanometer of very great delicacy is Sir 
William Thomson's mirror galvanometer, till recently 
used for signalling through submarine cables. Its 
general appearance is given in Fig. 19. The readings 




Fig. 19. Thomson's Mirror Galvanometer. 

are made by means of a very small light mirror of 
silvered glass fastened to the magnetic needle. 
"When the instrument is in use, a beam of light is 
made to fall on the mirror from a lamp, and is reflected 
with every movement of the needle to a different point 
on the scale placed opposite, thus indicating exactly 
the amount of deflection. The curved metallic piece 
M is a controlling magnet. 



chap, in MAGNETIC EFFECTS OF CURRENT 171 

In order that a galvanometer should be able to 
perform its object, viz. afford a means of accurately 
measuring the strength of currents, it is necessary in 
every instrument to ascertain the exact deflections 
of the needle corresponding to definite currents. 
When this is known, a basis of comparison is pro- 
vided, because the same instrument under the 
same conditions will always show the same 
deflection for the same current. When the experi- 
ments and calculations necessary to determine the 
deflections of the needle of any particular galvano- 
meter for various currents have been gone through, it 
is said to be calibrated. It is calibrated absolutely if 
the actual currents in amperes 1 producing the 
different deflections are known ; and relatively if only 
the connection between these deflections and the 
relative current-strengths is determined. 

The marked effects produced by electric currents 
on the magnetic needle, give a very clear indication 
that there is some close relationship between such 
currents and magnetism. A yet more striking proof 
of this is, however, afforded by the fact that an 
electric current itself possesses magnetic properties. 
The simplest way of proving this is by passing a 
battery current through a piece of straight copper 

1 The ampere is the unit of current strength, and is denned on 
p. 203. 



172 CURRENT ELECTRICITY part hi 

wire, and then approaching iron filings to it. The 
filings at once set themselves at right angles to 
the wire and cling round it, continuing to do so 
as long as the current passes ; thus showing that 
the wire has acquired for the time being the power 
of attracting magnetic substances, if they come 
within its range of influence. In fact, it produces a 
magnetic field. 

If instead of a straight wire a wire curved into a 
single loop, as in Fig. 20, be used, and the current 

passed through that, the 
magnetic field is now 
enclosed within the loop 
and coincides with its 
FlG - 20 * edges. In fact, such an 

arrangement as this is exactly like a magnetic shell, 
which we saw (p. 122) was a magnetised sheet of 
metal, one surface being entirely north-seeking and 
the other entirely south -seeking. If the observer 
be so placed as to look down on the loop, and the 
current be flowing through it from right to left as 
shown in the figure, i.e. in the same direction as the 
hands of a watch move, the upper surface of the 
loop and the space enclosed will be south-seeking. 
Eeferring to Ampere's rule, we should find that a 
man swimming with the current and facing towards 
the centre of the loop would be obliged to keep his 




:hap. in MAGNETIC EFFECTS OF CURRENT 173 

eft side down, consequently the north-seeking pole 
)f a magnet would turn itself downwards through 
such a loop. If, however, the current were flowing 
rom left to right, i.e. in the opposite direction to that 
n which the hands of a watch move, the upper 
surface of the loop and the enclosed space would be 
north-seeking. In swimming with the current and 
facing towards the centre, Ampere's man would have 
keep his left side v.p. Transforming him into a 
magnet, we should find the north -seeking pole 
urged upwards. 

A curious and interesting experiment may be 
made to illustrate these facts by means of De la Eive's 
floating battery, which consists of a strip of zinc and 
a strip of copper passed through a large cork and 
set floating in a vessel containing acidulated water. 
If the metallic strips be connected by a stout copper 
ring, and a bar magnet held towards it, the ring 
will be attracted or repelled according to the 
pole presented. If the north-seeking pole of the 
magnet be held towards the south-seeking face of the 
ring, the latter will be attracted, and will thread 
itself on to the magnet quite up to the centre. If 
the south-seeking pole be presented to the south- 
seeking face of the ring, the latter will be repelled, 
and if nevertheless forced to pass on to the magnet, 
will as soon as let go rapidly unthread itself, turn 



174 CURRENT ELECTRICITY part hi 

round so as to present its north-seeking face, and 
then re-thread itself up to the centre of the magnet 
as before. This is exactly how a magnetic shell 
would behave under similar circumstances, supposing 
that a hole were pierced through its centre to allow 
of its passing on to the magnet ; and in fact every 
closed voltaic circuit (of which the loop or ring 
we have been considering is an instance,) is in all 
respects equivalent to a magnetic shell of the same 
dimensions. It attracts and repels according to the 
same laws, and moreover, if placed itself in a magnetic 
field, it experiences just the same influence as the 
shell would do. We have therefore here a most 
striking illustration of the close relationship between 
magnetism and current electricity, and it will hardly 
surprise the reader to hear that according to the 
most modern theory they are in fact identical, only 
in the latter the flow takes place from one point to 
another, and may be compared to that of a river, 
whereas in magnetism the movement is one of 
rotation like the motion of water in a whirlpool. 
According to this theory, a magnet (which consists 
of a number of infinitesimal magnets) has a separate 
current of electricity circulating round each one 
of its molecules, and these currents when a perfect 
state of magnetisation is reached, and all the 
molecules are set end to end, are parallel to each 



shap. in MAGNETIC EFFECTS OF CURRENT 175 



other. Without entering into details which might 
be found tedious and complicated, it is sufficient to 
state that their effect on external space and objects, 
is exactly the same as though a current were circu- 
lating round the outside of the magnet. This is 
because only the currents belonging to the surface 
molecules are free to act externally at all, those in 
the interior being neutralised by their action on each 
other. The theory of these molecular currents is due 
to Ampere, and they are called by his name, 
amperian currents. It is the case that they do 
satisfactorily explain magnetic phenomena, but it is 
not probable that they are called into existence by 
the act of magnetisation. They are most likely 
already present in magnetic, and in fact in all sub- 
stances. Magnetisation merely renders their presence 
sensible externally, by setting them in a parallel 
direction through altering the position of the mole- 
cules. Those substances therefore whose internal 
structure does not lend itself to such a change, or 
only with great difficulty, are not capable of magnet- 
isation in the ordinary sense of the term. It must 
be remembered, however, that when a sufficiently 
powerful external force is exerted, all substances do 
feebly show signs either of magnetic or diamagnetic 
phenomena, and the latter also are explicable by 
means of Ampere's theory, 



176 CURRENT ELECTRICITY part hi 

That the production of rotation is a characteristic 
of magnetism can be very easily proved. One simple 
and striking experiment is described by Dr. Oliver 
Lodge in his Modem Views of Electricity} A long 
piece of gold thread is suspended in close proximity 
to an upright bar magnet, and a current passed 
through the thread. The latter ' immediately begins 
to coil itself round the magnet, half of it round the 
north - seeking pole, and the other half round the 
south-seeking pole, in such a manner that the two 
halves form a common spiral. If the gold thread 
were exchanged for a stiff wire, and a flexible magnet 
used, the magnet would then coil itself round the 
wire. In fact, it is proved that a single magnet pole 
would, if free to move, continually revolve round an 
electric current, and that an electric current would in 
the same way revolve round a magnet pole. As we 
know, however, it is impossible to obtain a magnet 
with one pole, it must always have two, and there- 
fore a rigid magnet and a rigid conductor cannot 
possibly show this movement of rotation. All they 
can accomplish is to place themselves at right angles 
to each other, as we have seen the magnetic needle 
invariably tends to do when placed over, under, or 
near a wire through which an electric current is 
passing. 

1 P. 135. 



chap, in EFFECTS OF THE CURRENT 177 

Liquid as well as solid conductors can be made to 
rotate under the influence of magnetism. If a vessel 
containing acidulated water be placed over a powerful 
bar magnet and electrodes immersed in it, one at 
the centre of the vessel and one at the edge, the 
liquid wall begin to rotate, and that so forcibly as 
very likely to cause it to splash over the sides of 
the vessel when a current is sent through the Kquid. 



CHAPTER IV 

ELECTRO-MAGNETS 

Definition of the term — Discovery of the way of making electro- 
magnets — Solenoids — Manner of insulating the coils of an 
electro-magnet— Shapes of electro-magnets — Way of winding 
the coils of a horseshoe electro -magnet — Position of north- 
seeking and south - seeking poles — Formation of consequent 
poles — Core of an electro -magnet takes time to become 
magnetised — Magnetic strength of electro - magnets — Coils 
with a number of turns only appropriate in a circuit of high 
resistance — Kind of metal used for the coils a matter of 
indifference — Reason of the importance of the iron core. 

From the observations made at the close of the pre- 
ceding chapter it will be seen that all magnets may 
perhaps be electro-magnets, because without electricity- 
it is probable that neither magnets nor magnetism 
would exist The term is not used, however, in this 
general sense, but refers exclusively to bars of iron 
or steel made into magnets by being enclosed in a 
spiral coil of wire through which an electric current 
is caused to pass. Steel treated in this way becomes 
permanently magnetised, but a bar of soft iron only 



CHAP. IV 



ELECTRO-MAGNETS 



179 



retains the whole of its magnetism while the current 
lasts. Consequently, this being a great practical 
convenience, it is soft iron which is almost invariably 
used for the " core," as it is called, of electro-magnets. 
The amount of magnetism which the core retains 
when the current ceases, is so faint that it seems 
hardly worth noticing. Nevertheless, as we shall 
hereafter find, this feeble " residual magnetism " has 
been made to yield the most important practical 
results, and it must not therefore go unmentioned. 

The principle of electro -magnets was known as 
far back as 1820, when Arago and Sir Humphry 
Davy independently discovered that a bar of iron or 
steel could be magnetised, as shown in Fig. 21, by 




Fig. 21. — Soft iron bar, magnetised by being placed within a wire spiral 
conveying the current from a single cell. 



being enclosed in a wire spiral through which an 
electric current was caused to circulate. The first 
practical electro -magnet was, however, made and 
exhibited by William Sturgeon in 1825, and he is 

13 



180 . CURRENT ELECTRICITY paiit hi 

therefore justly regarded as the inventor of this most 
useful and important appliance of electro-magnetism. 
Though a soft iron bar is always used in electro- 
magnets, it is not indispensable. A wire spiral 
without a core will also acquire magnetic properties, 
though never to such an intense degree. It is then 
called a solenoid, and behaves like a bar magnet, 
setting itself in the magnetic meridian if freely 
suspended, and having of course a north - seeking 
j> and south - seeking 

pole, which have the 
power of attracting 
HSTfrfflfif J '^RTSTfilP and repelling other 
FlG * 22, magnetic poles, and of 

attracting and being attracted by magnetic substances. 
Fig. 22 represents a solenoid arranged for suspension. 
In electro-magnets care must be taken that each 
coil of the wire is separated from the next and from 
the iron core, for if contact takes place at any point, 
the current passes from one coil to another instead of 
round each coil, and its effect is thus weakened ; for, 
as has been already stated in a previous chapter, the 
magnetising effect of the current is increased by 
increasing the number of coils in the wire, at least 
up to a certain point. 1 The insulation of the coils 
from each other and from the core is effected by 
1 See p. 168, note. 



CHAP. IV 



ELECTRO-MA GNETS 



181 



covering them with silk or cotton thread, the latter 
dipped in melted paraffin wax, or with a thin coating 
of gutta-percha, and they are wrapped as closely 
round the core as can be managed without weaken- 
ing the insulation. This is done to avoid the increase 
in resistance, which the greater length of wire 
required for wide coils would give rise to. The ends 
of the core always protrude beyond the coils. 

Electro-magnets, like permanent magnets, may be 
of any shape, but the most usual are the bar and the 

horseshoe. Kg. 
23 represents the 
latter, in which 
two coils of wire 
are used, leaving 
the central part 
of the magnet 
bare. In order 
that with this 
arrangement one 
pole may be north- 
seeking and one 
south-seeking, the 




Fig. 23.— Horseshoe Electro-magnet with keeper 
and hook for suspending weights. 



wire must be wound so that if the magnet were 
straightened out the coils would all follow the same 
direction. At whichever end the current enters the 
coils, the north-seeking pole is always that where it 



182 CURRENT ELECTRICITY part hi 

flows round them in the opposite direction to the way 
in which the hands of a watch move, and the south- 
seeking pole that at which it flows in the same 
direction. If the wire is coiled irregularly, at every 
change of direction a consequent pole is formed, just 
as happens in the case of an irregularly-magnetised 
ordinary magnet. 

The core of an electro-magnet takes time to become 
magnetised, partly because an electric current does 
not (as has already been stated, p. 151) attain to its 
full strength at once, and partly because of the 
transient inverse induction currents started in the 
core itself, when the magnetising current commences 
to flow through the coils (see p. 195). 

Augmenting the strength of the magnetising 
current, and augmenting the number of convolutions 
in the coil of an electro-magnet, alike increase the 
magnetising power of the latter. In fact, for low 
intensities of magnetisation the amount of magnetism 
is approximately proportional to the product of the 
current into the number of convolutions of wire, 
but after a certain point is reached this ceases to be 
true, because as the magnetism becomes stronger and 
stronger it increases less and less slowly with the 
product, so that beyond a certain point a very large 
increase in the product is necessary, in order to give 
rise to a small increase in the magnetism. Never- 



chap, iv ELECTRO-MAGNETS 183 

theless, as this small increase does take place, it 
cannot, strictly speaking, be said that such a thing 
as saturation exists in the case of an electro-magnet. 
Other considerations besides the intensity of the 
magnetising force to which it is subjected, affect the 
amount of magnetism which the core of an electro- 
magnet can acquire. The quality of iron of which 
the core is made, its shape, length, and thickness are 
all of importance. It must be remembered also, that 
since the resistance encountered by the magnetising 
current increases with the number of coils, it would 
be a mistake to include an electro - magnet with a 
great many coils in a circuit of otherwise low resist- 
ance, because the total resistance would be thereby 
so much increased as to weaken the current. A few 
turns of stout wire would in this instance answer 
the purpose better, whereas in a circuit which 
already has a high resistance an electro-magnet with 
many coils of fine wire is preferable. 

The wire used in the coils need not be of any 
particular metal. Copper is very often chosen, but 
for this purpose it has no special merit except its 
small specific resistance, as neither the material nor 
the thickness of the wire produces any effect on the 
strength of the electro-magnet. The important thing 
is that a sufficient quantity of electricity per second, 
should be carried sufficiently often round the iron 



1 84 CURRENT ELECTRICITY part hi 

core, to produce a magnetic field of the required in- 
tensity between it and them. For this purpose, when 
stout wire is used, a few turns will suffice, because 
in this case a considerable quantity of electricity will be 
carried round the coreof the electro-magnet in one con- 
volution, whereas in the case of fine wire a great many 
turns are necessary, since one convolution only suffices ' 
to carry a small quantity of electricity round the core. 
It is of interest to know why the introduction of 
a soft iron core into a wire spiral should so greatly 
increase its magnetic strength. The explanation is 
to be found in the fact of the alteration that takes 
place in the direction of the lines of force. These 
in an ordinary steel bar magnet run from end to end, 
and round outside from one pole to another (see Fig. 
11, p. 125). In a solenoid (without a core) very 
few of them do this, they nearly all remain as closed 
curves round the wire, each separate coil of which 
acts like a magnetic shell. When the core is intro- 
duced, on account of the high inductive power of 
iron, most of the lines of force in the solenoid are 
compelled to alter their direction and follow that of 
those existing in the iron itself, which run through 
the length of the iron and back from pole to pole as 
in the case of the steel magnet already described. 
Consequently the strength of the poles, being thus 
reinforced, is very greatly increased. 



CHAPTEE V 

ACTIONS OF CURRENTS UPON CURRENTS — INDUCTION 

CURRENTS 

Mechanical reaction of conductors which are conveying electric 
currents — Due to attraction and repulsion between the currents 
— Ampere's laws — Ampere's table — Further laws — Induction 
of one current by another — Primary and secondary coils — Direct 
and inverse currents — Induction of currents by magnets — 
Self-induction — Its effect on the primary current — Contact- 
breakers — High electro - motive force of induction coils 
— Ruhmkorffs coil — Sparks from induction coils — The aureole 
— Effects obtained by means of Geissler's tubes — Effect of a 
magnet on luminous discharge through rarefied air — Induction 
currents in solid masses of metal — Lenz's law — Experiment 
with metal disc suspended between the poles of two electro- 
magnets — Currents of the higher order. 

Hitherto our observations have been confined to the 
magnetising effects of an electric current, but another 
equally important fact demands attention. It is that 
electric currents act and react on each other, 
causing movements in the conductors conveying 
them. These movements are due to the mutual 
attractions and repulsions ; between the currents, 



1 86 CURRENT ELECTRICITY part in 

for flowing electricity, like electricity at rest, exhibits 
these phenomena, though they are governed by 
entirely different laws, first discovered and studied 
by Ampfere. 
He found — 

I. That currents conveyed by parallel wires attract 
each other if following the same, and repel each other 
if following different directions. 

II. That currents conveyed by wires which are 
inclined to each other at any angle, are mutually 
attracted if both flow towards or both flow from the 
apex of the angle, and mutually repelled if one flows 
towards and one from it. 

Ampfere devised an apparatus known as Ampere's 
table for observing the actions of currents on each 
other. It consists of a stand with double supports, 
upon which wire conductors of different shapes may 
be suspended in such a way as to allow them to 
rotate, and at the same time connected to a battery, 
so that a current may be passed through them and 
the behaviour of the various portions of wire with 
regard to each other be observed. By the help of 
this apparatus Ampfere showed that two parts of a 
circuit in whatever relative position they may be, 
experience a force tending to set them in such a 
direction as to enable the currents they convey to 
flow in the same path; also that a wire doubled 



chap, v CURRENTS UPON CURRENTS 187 

back on itself, so that the current takes a return 
path close to the one it was following before, does 
not exert any external force; and further that a zigzag 
wire exhibits the same magnetic influence over a 
not very near portion of the circuit as a straight one. 
Ampere also demonstrated that a conductor never 
experiences a force tending to move it in the direction 
of its own length, because the attractions and re- 
pulsions between currents always act at right angles 
to the currents themselves, tending therefore to 
make them revolve round each other. 

Since a mutual action exists between currents and 
currents and between currents and magnets, it is not 
surprising that under certain circumstances one 
current should be able to induce, i.e. bring into 
existence, another, and that magnets should also 
possess the same power. In their case it is exerted 
whenever a magnet is moved about in the neighbour- 
hood of a closed circuit, or when the circuit itself is 
moved in or across a magnetic field. In the case of 
currents, a current whose strength is changing induces 
a secondary current in any conductor near it ; and 
currents produced in either of these ways are said to 
be caused by electro-magnetic induction and are called 
induction currents. Their discovery is due to Faraday. 

In order to show the induction of currents by 
currents, two coils of wire are necessary, of which one 



i88 



CURRENT ELECTRICITY 



PART III 



is usually large enough to allow of the other being 
inserted into its hollow. This is merely for con- 
venience sake, however, as the relative size of the 
two coils is quite immaterial, so far as the generation 
of induction currents is concerned. The small coil, 
which is called the 'primary and is made of stoutish 
wire with few turns, is connected to a battery, and 
the large coil, in which the wire is fine and often 
coiled many thousand times, is called the secondary} 
and connected to a long coil galvanometer, as shown 
in Fin:. 24. 




Fig. 24.— P, primary coil ; S, secondary coil ; C, battery cell ; 
G, position of galvanometer. 

When the battery current is passed through the 
primary coil and the latter inserted into the hollow 
of the secondary, the galvanometer needle indicates 

1 The result of having stout wire with few turns in the primary, 
and fine wire with many turns in the secondary coil, is that a large 
current of low electro-motive force induces a comparatively small 
current of very high electro-motive force. 



chap, v CURRENTS UPON CURRENTS 189 

a momentary current in the opposite direction to that 
in the primary coil, and the same effect is produced 
if the current starts in the primary while it is lying 
in the hollow of the secondary. When the former is 
withdrawn, or when circuit is broken, while it is lying 
in the hollow of the secondary, another current is 
indicated in the latter in the same direction as that 
in the primary. This is called a direct current, and 
the former an inverse current. Inverse currents are 
produced in the secondary coil whenever a current in 
the primary coil begins, increases in strength, or 
approaches nearer; direct currents in the secondary 
coil occur whenever that in the primary ends, decreases 
in strength, or recedes. Neither inverse nor direct cur- 
rents ever occur except when a current in the primary 
starts or stops, or when one of the coils is moved (for 
moving the secondary nearer to or farther from the 
primary, produces the same effect as moving the 
primary itself), and their duration is only momentary. 
To show the induction of currents by magnets, it 
is merely necessary to replace the primary coil of 
Fig. 24 by a bar magnet. It will be found that when- 
ever the magnet is inserted in, or approached to the 
hollow of the remaining coil, a current is produced 
in one direction, and a current in the opposite direc- 
tion whenever the magnet is taken out or withdrawn 
to a greater distance. 



igo CURRENT ELECTRICITY part hi 

Besides the induction of one current by another, 
there is also the induction of a current on itself, called 
more shortly self-induction, to which brief reference 
was made in a former chapter. By it is really 
meant, that if two portions of the same circuit are 
placed side by side, the sudden commencement or 
cessation of a current in one portion, tends to induce 
a momentary current in the other, just as if the two 
portions belonged to separate circuits. Thus suppose 
we have a wire doubled back on itself, as in Fig. 25, 
. — >» . — > and the primary current 

♦ ■ ; ; flowing in the direction 



Fig. 25.-Diagram to illustrate Self- indicated by the single 

induction. barbed arrows, at the 

commencement of such a current there would be a 
tendency to induce a momentary inverse current 
flowing in the direction of the double -barbed arrows ; 
and at the cessation of the primary current a tend- 
ency to induce a momentary direct current, flowing 
of course with the single-barbed arrows. 1 This induc- 
tion has the effect of weakening a current at its 
start (thus delaying its growing to its full value), 
and strengthening it at its cessation, which is thus 
retarded. In fact, since the induction of the current 

1 In a simple circuit, such as that represented in the figure, 
there would be very little self-induction however, whereas in a 
circuit coiled many times on itself, there would be a great deal. 



chap, v CURRENTS UPON CURRENTS 191 

in one part of a circuit, takes place on another part 
of the same circuit across the intervening medium, 
energy is transferred to the latter while the circuit is 
closed, and the current remains constant in strength, 
but is given back to the circuit again to produce 
the "extra current" on the stoppage of the main 
current. This is the reason why sparks are obtained 
on breaking circuit. They are mucli more brilliant 
if a coil of many turns be included in the circuit ; 
and if the coil contains a soft iron core, this again 
increases the sparking power. There are various 
automatic contrivances called contact-breakers, or in- 
terrupters, used for making and breaking circuit with 
regularity and rapidity, but a detailed description of 
them is unnecessary for the present purpose. 

Induction currents have an enormously high 
electro -motive force, and very striking effects can 
consequently be produced by them. These are shown 
in a marked and powerful way by the induction coil 
or inductorium, often known under the name of 
Ruhmkorff's coil, as that inventor did much to per- 
fect it. Its most important parts are of course a 
primary and secondary coil, placed one within the 
other, and the former connected with a battery and 
containing a core of straight soft iron wires. Under 
the coils there is a condenser, which is placed within 
a flat wooden box and consists of sheets of tinfoil, 



192 CURRENT ELECTRICITY part hi 

separated by sheets of paraffined paper, each alternate 
piece of tinfoil being electrically connected, so that 
the whole set forms two series corresponding to the 
inner and outer coatings of a Leyden jar. A con- 
tact-breaker and a commutator or key, whose use is 
to reverse the direction of the battery current when- 
ever the operator chooses, complete the apparatus. 
The wires from the primary coil are, as has already 
been said, connected with the terminals of a battery, 
and those from the secondary with the condenser, the 
use of this latter arrangement being that the spark 
on breaking circuit, may be mitigated by lessening 
the amount of extra current in the primary coil, some 
of the electricity flowing into the condenser, instead 
of to the point where the break is made. 

The sparks from an induction coil are extremely 
powerful, and often attain a great length. From 
eighteen to twenty inches is not at all unusual with 
a large instrument, and sparks a metre long have 
been obtained from some of its most modern forms. 
These sparks are not exactly the same as those ob- 
tained from an electrical machine or a Leyden jar ; 
for besides the spark proper, there is an aureole or 
glow which surrounds it, and which by the help of a 
suitable apparatus can be detached from it, thus 
showing that the two appearances are caused by 
different discharges. Experiments of various kinds 



chap. V CURRENTS UPON CURRENTS 193 



seem to prove that the sharply-defined spark is analo- 
gous to that given by a battery on breaking circuit, 
and that the aureole is caused by the quiet combination 
of the opposite kinds of electricity in the same manner 
as in a galvanic current. 1 The sparks from induction 
coils are specially suitable for passing through Geiss- 
ler's tubes, and are very often used for this purpose 
in preference to those from an electrical machine. 
Very beautiful luminous effects are then obtained, 
especially if the tubes are made of uranium glass, 
or contain a solution of quinine or other " fluorescent " 
liquid. By fluorescence is meant the property pos- 
sessed by some substances of changing the colour of 
rays of light through altering their refrangibility ; 
phosphorescence is the power of becoming self- 
luminous, a property which can be conferred by the 
passage of an electric discharge, on that part of a 
very highly-rarefied gaseous medium near the nega- 
tive pole. The phenomena of fluorescence and 
phosphorescence, though somewhat complicated, pre- 
sent points of the highest interest and importance, 
and a hope seems even to be entertained by some 
of our leading scientists, that in the course of time 
discoveries will be made enabling us to produce for 
ordinary purposes artificial light of this description, 
which is unaccompanied by heat. In fact, experi- 
1 Electricity in the Service of Man, p. 199. 



194 



CURRENT ELECTRICITY 



PART III 



ments in this direction were made by Professors 
Ayrton and Perry so far back as 1879. 

An exceedingly interesting and instructive experi- 
ment can be made with a magnet on the luminous 
discharge in a Geissler's tube, such a discharge pos- 
sessing the properties of an electric current so that it 
deflects the magnetic needle, and is itself capable of 
being acted on by a magnet. Pig. 26 will enable this 

experiment to be under- 
stood. A soft iron bar B 
is enclosed in an exhausted 
glass vessel V, and is sur- 
rounded at the lower end 
by a metallic ring K, and 
carefully insulated. The 
terminals of a battery are 
then connected, one with 
the upper end of the 
apparatus and the other 
with the metal ring, and 
immediately a sheaf of 
luminous rays descends 
towards the ring, sur- 
rounding the soft iron bar. 




Fig. 26. 



If while the luminous discharge is thus passing, one 
pole of a magnet be held under the vessel, the iron 
bar becomes magnetised and the rays of light begin 



chap, v CURRENTS UPON CURRENTS 195 

to revolve round it, thus giving a striking proof both 
of the rotary tendency of the magnetic forces and of 
their effect on an electric discharge. 

It must not be supposed that induction currents 
show themselves only in wires. Solid bars or masses 
of metal of any shape are susceptible of them, and a 
magnet moved in the neighbourhood of a lump or 
plate of metal, or the starting or stopping of an electric 
current near it, induces in it currents which, owing to 
the resistance they encounter, very rapidly transform 
their energy into that of heat, and while they last 
tend to stop the motion of the magnet which gave 
rise to them. This they do in accordance with a 
law, known as Zenz's law from the name of its formu- 
lator, by which all induced currents flow in such 
a direction that their reaction tends to stop the 
motion producing them. They thus, in fact, offer a 
mechanical resistance, which is experienced by any 
conductor constrained to move across the lines of 
force in a magnetic field. A curious and striking 
instance can be given of this by suspending a 
metal disc by a twisted thread, between the 
poles of two powerful electro -magnets. While the 
magnets are inactive the disc revolves rapidly 
through the untwisting of the thread, but directly 
the current passes through the coils the disc stops 
dead, and if forcibly compelled to rotate, grows 

14 



196 CURRENT ELECTRICITY part in 

rapidly hot, showing how powerful is the resistance 
encountered. 

It has been discovered not only that primary 
currents give rise to secondary currents, but also that 
the latter are themselves able to induce currents in 
closed circuits near them, and not in any way con- 
nected with the primary. Thus, suppose that near 
the secondary coil a third coil is placed, and near 
this a fourth. When a current commences in the 
secondary coil an inverse current is induced in the 
third coil, and a direct current on its cessation. 
These currents in the third coil give rise in like 
manner to momentary currents in the fourth coil, and 
the process may be almost indefinitely repeated. 
Currents thus induced by the action of that in the 
secondary coil, are called currents of the higher order, 
and are said to belong to the third, fourth, or fifth 
order, etc., according to the remoteness of their gen- 
eration from the secondary. 



CHAPTEE VI 

PRACTICAL UNITS OF MEASUREMENT FOR ELECTRIC 

CURRENTS 

Ways of measuring electric currents — Need of units of measure- 
ment — C.G.S. units those on which "absolute" electro-mag- 
netic units are founded — "Practical units" those in common 
use — Recapitulation of what there is to measure in electric 
currents — Electro-motive force — Resistance — Strength — Quan- 
tity — Names and definitions of practical electro-magnetic units 
— Other units derived from these to express larger and smaller 
quantities — Necessity for a unit of capacity — The Farad and 
Microfarad. 

Electric currents can be measured in various ways 
— by their influence on the magnetic needle, 1 by the 
amount of chemical action they give rise to, 2 and by 
their heating effects. In order to measure them, how- 
ever, we must have some scale of measurement to go 
by. We need, for instance, a definite unit in order to 
calculate the strength of any given electric current, just 
as we need a definite unit of time so as to calculate 
the number of hours in a day or of days in a year. 
1 See chap. iii. 2 See chap. ii. 



198 CURRENT ELECTRICITY PAPwT hi 

All physical quantities, such as force, volume, 
velocity, etc., can be expressed in terms of length, 
mass, and time, and each of these requires a unit of 
its own on which to base its measurement. In 
science the 'almost universally adopted units are — 
for length the centimetre, for mass the gramme, and 
for time the second. The centimetre is 0*3937 (con- 
siderably less than two-thirds) of an inch, the gramme 
equals rather more than fifteen grains troy, and the 
value of the second is practically known to every 
one. The system of units derived from these as 
fundamentals is called after their initials the C.G.S. 
system, and the " absolute " electro-magnetic units, 
like all absolute derived units (such as those of 
area, volume, velocity, etc.) belong to it. With the 
absolute electro-magnetic units such a work as the 
present need not concern itself. It will suffice to 
obtain a tolerably clear idea of what are called the 
practical units, which are founded on the absolute 
units and are those in general use, some of the abso- 
lute units being too large and some too small for 
ordinary purposes. 

Before giving a list of these practical units, which 
are named after various eminent men whose dis- 
coveries in electrical science have been specially 
notable, it will be w^ell to recapitulate exactly what 
there is to measure in an electric current. 



chap, vi PRACTICAL UNITS OF MEASUREMENT 199 

In the first place there is the electro-motive force 
(nearly always indicated by the letters E.M.E.) 
without which, as has already been stated in Chap. 
I, p. 143, no current can exist, and on which the 
power of the current to overcome resistance depends. 

Next in order to electro-motive force comes resist- 
ance, defined in a former chapter as the opposition 
offered to the passage of electricity through any 
material substance, and also as the ratio of potential 
difference to current strength, which in a homo- 
geneous conductor at a given temperature is constant. 
Eesistance of course varies inversely as conductivity. 
The better the conductor the less the resistance, the 
worse the conductor the greater the resistance. 
Every substance has its own specific resistance, just 
as every substance has its own specific heat. 1 In 
the case of metals the order of least resistance for 
electricity is very nearly the same as that for heat, 
so that silver, which is the best heat conductor, is 
one of the best electrical conductors also, and mercury 
one of the worst. In the same substance the re- 
sistance varies with difference of temperature ; in 
metals increasing with increase of temperature, in 
insulators, decreasing. Other causes also affect re- 

1 The specific heat of any substance is the amount of heat re- 
quired to raise it 1° C. in temperature. The specific resistance is 
the "resistance in 'absolute' C.G.S. units (i.e. in thousands 
millionths of an ohm) of a centimetre cube of the substance." 



200 CURRENT ELECTRICITY part in 

sistance ; it increases with the length of wires, and 
decreases with their thickness. The resistance of a 
stout metal rod would be nothing in comparison with 
that of a piece of telegraph wire of the same length, 
and that of the latter, if only 100 yards long, would 
be a negligible quantity compared to what it would 
attain if the wire were 100 miles long. 

Eesistance is usually measured by comparison 
with that offered by standard coils of wire whose 
resistance at a given temperature is known. They 
are called resistance coils, and are wound double, so 
that their interaction may preserve them from all 
external electric and magnetic influence, and are 
usually made of German silver, whose resistance 
changes little with change of temperature, or of an 
alloy of silver and platinum, which possesses the 
same property, or of platinoid. Alloys of metals of 
course have different resistances from the metals 
themselves, and even the slightest impurity affects 
them in this respect. 

By the total resistance of a circuit is meant that 
offered by the whole circuit, and by the internal 
resistance that which is encountered in the generat- 
ing source of the current— as, for instance in the 
battery cells of a galvanic circuit. 

Besides electro-motive force and resistance, there 
is the strength of current to be measured, which is 



chap, vi PRACTICAL UNITS OF MEASUREMENT 201 

evidently dependent on them both. Given a high 
electro-motive force and a low resistance, we shall 
get a strong current, just as given water running 
through a pipe at high pressure and with no impedi- 
ment, we shall get a powerful outflow at the tap. 
But given the same electro-motive force and a high 
resistance, we shall get a weak current, just as we 
should get a feeble outflow of water at the tap in 
spite of the former being at high pressure, if it had 
to flow through a partially choked pipe. In fact, 
strength of current means the quantity of electricity 
flowing past a given point in a given time, and there- 
fore it is necessary to be able to measure, and to have 
a unit of measurement for 

Quantity of electricity. Electro -motive force is 
indeed independent of it, but since strength of current 
is actually the quantity of electricity flowing per 
second past a given point in a conductor, this 
strength can be augmented in two ways, either 
by increasing the electro -motive, i.e. the driving, 
force, or by making the current larger. The 
pressure of falling water in a pipe is not increased 
by increasing the size of the pipe, but by increasing 
the difference of level ; yet, other things being 
equal, the strongest stream of water will issue 
from the largest pipe simply because it is the 
largest and holds the most water. In the same 



202 CURRENT ELECTRICITY part ill 

way electro - motive force is not increased by 
doubling or trebling the quantity of electricity con- 
veyed by a given conductor, but by doubling or 
trebling the difference of potential between the two 
ends of that conductor ; yet the electro-motive force 
being the same, a stronger current will flow through 
a thick than through a thin wire, merely because 
its- thickness allows of the passage of a greater 
quantity of electricity through the same distance in 
the same time. 

Having thus described what there is to measure 
in an electric current, the list of units can now be 
given. 

The volt (derived from Volta) is the unit of 
electro-motive force. 1 It is defined as the difference 
of potential that must be maintained at the ends of 
a wire of one ohm resistance, so that a current of 
one ampere may pass through it. 

The ohm is the unit of resistance, and is equivalent 
to the resistance of a column of mercury one milli- 
metre square and 106 centimetres in length, at a 
temperature of 0° Centigrade. One mile of ordinary 

1 For practical purposes some particular cell (a standard Darnell's, 
for example), whose electro-motive force is as nearly as possible 
one volt, and whose constancy can be depended on, is often used 
as a standard of electro-motive force ; but this does not form the 
scientific standard of the volt, which is not dependent on the 
electro-motive force of any cell or chemical combination. 



chap, vi PRACTICAL UNITS OF MEASUREMENT 203 

iron telegraph wire has a resistance of from 10 to 
20 ohms. 

The ampere is the unit of strength of current, and 
may be defined as a current strong enough to deposit 
0.000329 (329-millionths) of a gramme of copper per 
second on one of the plates of a copper voltameter. 

The eoulomh is the unit of quantity, and means 
that quantity flowing in one second past the cross- 
section of a conductor conveying an ampere. 

Since quantities almost indefinitely greater and 
smaller than those signified by the above units have 
to be measured by electricians, prefixes are often 
used expressing one thousand or one million times 
more, or one-thousandth or one-millionth part, so as to 
avoid the inconvenience of writing and reading such 
enormous numbers as would otherwise be necessary. 
For instance, the currents in ordinary telegraphy are 
not measured by amperes, but by milli - amperes 
(or thousandths of an ampere), and the resistance in 
a good telegraph insulator not by ohms, but by 
megohms (millions of ohms). The electro-motive force 
of a lightning flash would be measured, if it could be 
measured, by mega-volts (millions of volts), and the 
strength of telephone currents by micro- (millionth) 
amperes. 

It will be understood that though these practical 
units have been mentioned only with reference to 



204 CURRENT ELECTRICITY part in 

current measurement, they can also be used for 
electro-static purposes. The electro-motive force of a 
Leyden jar (i.e. of the difference of potential between 
its two coatings), or of any condenser, would be 
measured in volts, just as would be the electro- 
motive force of a galvanic battery ; and the resistance 
of any conductor would be expressed in ohms. 

It is clear that the electro -static capacity of a 
conductor, i.e. the amount of electricity which, owing 
to its size, shape, and position, with reference 
to other conductors, it is capable of accumu- 
lating must be of great importance in much practical 
work, and therefore a unit of capacity needs to be 
added to those already named. It is called a farad 
(from Faraday) ; and a condenser, which must natur- 
ally be the standard of capacity, has a capacity of 
one farad, when a potential difference of one volt 
between its two sets of plates charges each of them 
with one coulomb. A condenser constructed of tin- 
foil and paraffined paper like that described in 
connection with induction coils is most frequently 
used in practical work, but if made on the scale of 
one farad as a unit it would be so enormous as to 
be almost impossible of construction, and quite 
unmanageable for all ordinary purposes if it were 
constructed. The practical unit of capacity is there- 
fore in reality the microfarad (one -millionth of a 



chap, vi PRACTICAL UNITS OF MEASUREMENT 205 

farad), and condensers are made graduated in micro- 
farads. Even then, for some purposes (such as 
" duplexing " submarine cables) condensers con- 
taining many thousand square feet of tinfoil are 
necessary. 



PART IV 

PRACTICAL APPLIANCES OF 
ELECTRICITY 

CHAPTER I 

MAGNETO-ELECTRIC AND DYNAMO-ELECTRIC MACHINES 
AND ELECTRO-MOTORS 

Number of practical electrical appliances in modern days — Mag- 
neto-electric machines — Pixii's machine — Clarke's machine — 
Wilde's machine — Siemens' cylindrical armature — Dynamo 
machines — Their principle — Origin of their name — Now in- 
cludes magneto-electric machines — Siemens' first self-exciting 
dynamo — Gramme machine — Principle of Gramme ring — Con- 
tinuous current machines — Alternate current machines — 
Various kinds of dynamos — Requisites for a good dynamo — 
Electro- motors — Their work the converse of that of dynamos 
— Powerful currents generated by dynamos — Currents of high 
E.M.F. used for electric lighting — Transformers. 

The practical appliances of electricity in our day are 
so numerous and so important that it is difficult to 
know with which to begim The electric telegraph is 



chap. I DYNAMO ELECTRIC MACHINES 207 

the oldest, and has already attained a familiarity 
which, in this instance, however, certainly does not 
breed contempt, for no later invention can surpass 
or perhaps even equal it, in the magnitude of its effects 
on the whole human race. Nevertheless, the more 
recent adoption of electric lighting and electric 
transmission of power gives them for the moment a 
greater prominence in the eyes of the world ; and 
since it is impossible to gain any notion of their 
working principles without some knowledge of what 
is meant by the three classes of machines whose 
names head this chapter, it will perhaps be as w r ell 
to commence with them. 

Priority of date belongs to the magneto-electric 
machines, the first of which was constructed in 1833 
by Pixii, a scientific instrument-maker in Paris. His 
apparatus consisted of a steel horseshoe magnet, which 
was made to revolve rapidly before two wire bobbins, 
so that its north -seeking and south -seeking poles 
passed alternately in front of them. Currents w T ere 
thus induced in the bobbins which changed direction 
at every half-revolution of the magnet, but which, 
owing to the way in which the bobbins were wound, 1 
would, if the coils were laid end to end, flow through 
both in the same direction at the same time. Wires 
connected the apparatus with an outer circuit, through 

1 See Part III. Chap. iv. 



208 



PRACTICAL APPLIANCES 



PART IV 




which the currents could be conducted and utilised 

for any desired ob- 
ject, and in order 
to obviate the (for 
many purposes) in- 
convenient alterna- 
tions of the cur- 
rents, a commutator 
(current reverser) 
was introduced, by 
means of which the 
currents in the outer 
circuit could be 
rendered continuous, 
i.e. obliged to flow 
always in the same 
direction. Their ex- 
tremely rapid suc- 
cession then enabled 
them for practical 
purposes to behave 
as one uninterrupt- 
ed current. Fig. 27 
enables the principle 
of Pixii's machine 




Fig. 27.— To illustrate the principle of Pixii's 
Machine. M, revolving horse-shoe magnet ; 
A B, soft iron cores surrounded by wire 
spirals. When S is approaching to and 
opposite A, and N is approaching to and 
opposite B, the induced currents flow in 
the direction indicated by the arrows. When 
S is approaching to and opposite B, and N 
is approaching to and opposite A, they flow 
in the direction opposed to the arrows. In 
each case there is a momentary cessation of 
current when one of the magnet poles con- 
fronts A and the other B, and a maximum 
of current when the magnet reaches the 
position C, so that it is at right angles with 
the plane of the paper. 



to be understood. In its construction, however, the 
magnet is placed below and not above the coils. 



chap. I DYNAMO MACHINES 209 

The drawback to Pixii's machine lay in the neces- 
sity (when a large size was required) of rotating an 
exceedingly heavy magnet, compelling the expenditure 
of much mechanical work, for which but a compara- 
tively small return in the shape of electrical energy 
was made. Clarke and other inventors conceived the 
idea of making the bobbins revolve before the magnet, 
instead of the magnet before the bobbins, so that the 
heaviest part of the apparatus should remain at rest ; 
and much saving of mechanical power was effected in 
this way. A small portable form of Clarke's machine 
is still frequently used for medical purposes. 

Magneto -electric machines were next improved 
by constructing them with electro as well as with 
permanent magnets. In Wilde's machine, one of the 
best of the earlier kind, the steel horseshoe magnets 
are quite small, and used merely to generate currents 
in a small electro-magnet of cylindrical shape, revolv- 
ing between their poles, and called an armature. The 
induced currents in this armature are then carried by 
means of connecting wires through the coils of two 
large fixed electro - magnets, known as the field 
magnets, between whose poles another cylindrical 
armature revolves. The currents thus obtained are 
exceedingly powerful; but a great drawback to 
Wilde's machine exists in the large amount of 
heat it generates, which not only rapidly weakens 



210 



PRACTICAL APPLIANCES 



PART IV 



the current, but makes its constancy a matter of 
impossibility. 

The cylindrical armature used in Wilde's and in 
many magneto -electric and dynamo machines was 
invented by Dr. Werner Siemens, one of the four 
celebrated brothers whose scientific discoveries and 
appliances are so justly famous. Its utility consists 
in the fact, that owing to its shape it is 
able to revolve in the most powerful part 
of the magnetic field, i.e. exactly between 
the poles, thus combining high efficiency 
with great economy of space. The wire 
in a Siemens armature is wound length- 
wise like thread in a shuttle, and is 
enclosed in an iron sheath open at the 
sides, as represented in Fig. 28. The 
poles are not situated at the ends of this 
armature (or electro-magnet), but on the 
two faces (P P) of the iron sheath which 
have not been cut away. At every half- 
revolution of the armature the polarity 
is reversed, that which was a north- 
seeking becoming a south -seeking pole, 
and vice versa. 



Fig. 28.— 

Shuttle-wound 

Siemens 

Armature. 



The principle of the dynamo machines, shortly 
stated, consists in utilising the residual magnetism 
left in the soft iron core of an electro-magnet as a 



chap. I DYNAMO MACHINES 211 

current generator. Nearly all iron retains a faint 
remnant of magnetism when once it has been highly 
magnetised; and therefore an electro-magnet, not 
externally excited, but rotating as an armature be- 
tween the poles of two much larger electro-magnets, 
first induces feeble currents in its own coil, and these 
being transmitted through the coils of the fixed mag- 
nets, render them also active, and consequently able 
in their turn to exalt the magnetism of the rotating 
armature by their reaction on it. The armature thus 
reinforced, itself induces and transmits more power- 
ful currents, to be again strengthened in their passage 
through the fixed or field magnets (thus named because 
their work is to form a powerful magnetic field for 
the armature to revolve in,) and by this system of 
continued action and reaction exceedingly strong 
currents can be obtained in an incredibly short time, 
and conducted into an outer circuit for use. 

The name of dynamos was originally given to 
machines worked on this principle, because at first 
sight they appear to owe their electrical energy more 
directly to the mechanical power expended in 
rotating the coils, than do the magneto- electric 
machines. Such is not in reality the case, however. 
Just as both kinds of machines derive their capability 
of generating electric currents from electro-magnetic 
induction, so also both kinds must have mechanical 

15 



212 



PRACTICAL APPLIANCES 



PART IV 



energy of some description, be it in the form of hand, 
horse, steam, or water power, expended on them in 
order to convert it into electrical energy ; and it has 
now become common to include them all under the 
name of dynamos, distinguishing the two classes as 
magneto and self-exciting dynamos. 




Fig. 29.— Siemens Dynamo (compound -wound). A, armature; M M, field- 
magnet coils ; P, iron pole piece attached to near magnet (that of the far 
magnet cannot be distinguished) ; C C, commutator and collecting brushes 
T T, terminal binding screws. 

The principle of the self- exciting dynamo was 
simultaneously discovered by Siemens and a 
partner in the Siemens firm, Hefner von Alteneck, 
and by Wheatstone and Varley. Since its first 



CHAP. I 



DYNAMO MACHINES 



213 



appearance the Siemens machine has undergone 
various improvements. One of the latest types is 
represented in Fig. 29. It consists of two powerful 
flat electro-magnets, between whose poles is a rotating 
armature, not made on the plan of the original 
shuttle-wound Siemens armature, but on the same 
general principle as the Gramme ring, which gives its 
name to another typical dynamo machine. 

The principle of this ring may be broadly under- 
stood by reference 
to Fig. 30. Sup- 
pose N and S to 
be the poles of 
a magnet between 
which a ring of 
soft iron, wound 
over with insul- 
ated copper wire, 
is revolving in 
the direction indi- 
cated by the large 
inner arrow A. Owing to the action of N and 
S, magnetism will be induced in the iron ring, 
and whatever portion of it is for the moment 
opposite N 1 will possess south -seeking, and that 




Fig. 30— To illustrate the principle of the 
Gramme Ring. 



1 Not exactly opposite, however. The poles of the rotating ring 
will in each case be a little further forward in the direction of 



214 PRACTICAL APPLIANCES part iv 

opposite S north-seeking magnetism, so that the ring 
itself acquires poles which remain stationary despite 
its own movement of rotation. In consequence of 
this, the currents induced by it in its surrounding 
coil will be such as to flow always in one direction 
between B, N", B', and in the opposite direction between 
B, S, B', as indicated by the small arrows placed near 
each convolution of the wire. The result of this will 
be a rise of potential from B past 1ST to B' through 
one half of the ring, and from B past S to B' through 
the opposite half of the ring, so that the point of 
highest potential will always be in that turn of the 
wire opposite B', and the point of lowest potential 
in that opposite B. A current will therefore flow 
through any conductor whose ends are respectively 
in contact with these points of highest and lowest 
potential, just as a current will flow through any 
conductor whose ends are connected to the opposite 
poles of a galvanic battery. In the actual Gramme 
machine the ring is not made of one circular bar of 
soft iron, but of a number of iron wires bent into the 
required shape, in order to avoid the greater amount 
of self-induction which would take place in the 
former case, and which on account of the resistance 
it entails and the consequent large heat-generation, 

rotation than the poles of the field magnets, owing to the iron 
requiring time to attain its maximum state of magnetisation. 



chap. I DYNAMO MACHINES 215 

causes a great waste of energy. 1 The armature coils 
also are not continuous, as in the figure, but divided 
into sections, which are connected to each other in 
series, every one being also connected to a separate 
copper strip forming a segment of the commutator. 
The necessary contact between the points of highest 
and lowest potential and the conducting wires is 
brought about by means of wire brushes, or in large 
machines by thin copper plates, which are situated 
at these points, and which as the ring revolves rub 
against the copper strips already mentioned as 
forming segments of the commutator. This last is 
rendered necessary by the fact that a change of 
direction in the current takes place at B and B' (see 
the reversed position of the arrows at these points), 
so that, unless special means were used to prevent it, 
the current sent through the outer circuit would be 
alternating. Through the action of the commutator, 
however, it is rendered continuous. The Siemens 
dynamo represented in Fig. 29 is also a continuous- 
current machine, but a large and important class of 
dynamos is made to produce alternating currents, an din 
these (in which of course no commutator is necessary), 
the direction of the current is constantly reversed, often 

1 No armature cores, in fact, are solid. They are all made of 
wires or of thin insulated sheets of iron, to reduce as much as possible 
what are known as the " eddy " or Foucault currents. 



2l6 



PRACTICAL APPLIANCES 



PART IV 



many hundred times in a second. Fig. 31 represents 
a simple form of continuous-current dynamo with a 
Gramme armature. 

It must not be supposed that the dynamos above 




Fig. 31.— Simple form of Dynamo with Gramme Armature. M, coil of field 
magnet ; P P, iron pole pieces ; A, armature ; C, commutator, B B, 
collecting brushes. 

named are those in exclusive use now. They have 
been selected for mention partly on account of their 
historical interest, partly because they are still in wide 
Besides the machines distinguished 



use and favour. 



chap. I DYNAMO MACHINES 217 

by having cylindrical and ring-shaped armatures, many 
are made whose armatures have the form of a drum, 
others that of a disc. All these are supposed to 
possess certain technical advantages, but the main 
principle of all is the same, and that is the one 
important thing for the non-technical reader to grasp. 
Beyond this he will readily understand that what is 
wanted to make a good dynamo is a powerful magnetic 
field, a low resistance in the electric circuit, a great 
facility of magnetisation (high magnetic permeability,) 
in the armature cores and any magnetic portion of 
the machine, and as little waste of energy as possible 
through heating. These objects are accomplished in 
various ways by various makers, and since it is 
impossible that all can be equally well attended to in 
all dynamos, the same kind will not be found equally 
suited for every purpose. Each special class of 
dynamo will be best adapted to some special kind of 
work, and experience alone can assign to each that 
for which it is most fitted. It may be mentioned, 
however, that the suitability of dynamos to any 
particular purpose depends very much on the manner 
in which the magnet coils are wound. This may 
be in one of three ways— " series/' " shunt," or 
\ compound." 

Series-wound dynamos send the whole of their 
armature current through the field-magnet coils, which 



218 PRACTICAL APPLIANCES part iv 

are connected in series with the main circuit. These 
dynamos are used for arc lighting in series, and some- 
times for charging accumulators. 

In shunt- wound dynamos the terminals of the field- 
magnet coils and those of the main circuit are 
separately connected to the collecting brushes, so that 
the two circuits are in parallel and not in series. 
The magnet coils being made of very fine wire, offer 
a much higher resistance than the main circuit, 
receiving in consequence but a small proportion of the 
current, they are therefore said to act as a " shunt." 
Dynamos of this kind are used for charging 
accumulators, and in some systems of incandescent 
lighting. 

Compound-wound dynamos have the great advan- 
tage of being self- regulating, because a constant 
potential difference is maintained at their terminals, 
so that the current in the outer circuit is inversely 
as the resistance. These dynamos are used for arc 
and incandescent lighting in parallel, (viz. in systems 
where the current does not flow from one lamp 
to another, but to all separately,) and in any case 
where much regulation is necessary. 

The third class of machines to be considered is 
that of electro -magnetic engines, or as they are 
usually called electro-motors. These, as their name 
indicates, do mechanical work by means of electricity. 



chap, i ELECTRO-MOTORS 219 

We have seen that dynamos have mechanical work 
expended on them in order that they may turn it 
into electrical energy. In electro-motors the process 
is reversed; they turn electrical into mechanical 
energy, and that with far less waste than occurs in 
the analogous case of a steam-engine turning heat 
into mechanical energy, so that as a motive power 
electricity might ere this have driven other rivals out 
of the field, had it not been for the great expense till 
recently entailed by its use. 

The constant and persevering efforts of inventors 
have therefore been directed as much to reducing the 
cost, as to improving the construction of dynamos 
and electro -motors, with how large a measure of 
success the wide-spread adoption of electric lighting, 
and to a lesser extent of electric traction, sufficiently 
prove, while the growing popularity of electrical 
engineering as a profession, shows with what con- 
fidence its future is regarded. 

It is not necessary to enter into any detailed 
description of the construction of electro-motors, as 
they are so similar to continuous-current dynamo 
machines, that any one of these can be used as an 
electro-motor, 1 only if it is so used, it must be driven 

1 Nevertheless it is usually found better in practice to make a 
machine to be used as an electro-motor specially for that purpose, 
as slight differences of construction can then be introduced which 
render it more efficient. 



220 PRACTICAL APPLIANCES part iv 

by a current from an external source, not by its own 
current. The same machine cannot be used at the 
the same time as a producer of both mechanical and 
electrical energy ; it will give out either, but not 
both, and one must first be given to it. Fig. 32 
represents a machine constructed purposely to be 
used as an electro-motor. 

The currents generated 
by dynamos may be either 
of very high electro-motive 
force or of great strength 
(many amperes), or both, 
according to the purpose 
for which they are xe- 
motor quired, and the conse- 

quent construction of the 
machine. Tor many purposes (such as electric 
lighting) it is preferable to use currents of a 
very high electro -motive force. This is much the 
same as employing a small volume of water at 
high pressure, instead of a large volume at low 
pressure ; and, in fact, the term " electric pres- 
sure" is very frequently used by engineers. It 
is important to bear in mind, however, that this 
is merely a convenient mode of expression, 
and does not assert (as in the case of water,) an 
ascertained fact. Currents of high " electric 




chap. I TRANSFORMERS 221 

pressure," i.e. of considerable electro - motive force 
(1000 volts and upwards), are attended with great 
risk ; for if due precaution is not taken, they are 
dangerous to life, and should therefore never be 
carried into private houses, nor is there as a rule 
any need to do so. For any ordinary illuminating 
purposes a current of 100 volts is amply sufficient ; 
and though along the main wires from a central 
lighting station, it may be necessary to have a 
current of 2000 volts or more, by means of trans- 
formers this can be reduced on entering a house to 
100 volts. Transformers are only an adaptation of 
induction coils in which the usual process is reversed, 
and instead of a large current of low electro-motive 
force inducing a small current of high electro-motive 
force, the exact opposite takes place. Transformers 
can, however, only be made use of with alternating 
currents, for it will be remembered that secondary 
currents never arise except when the strength of the 
primary is changing. Consequently, if the latter 
were continuous, secondary currents and any apparatus 
depending on them would be impossible. Where 
transformers are used, the electric-lighting circuit of 
each house is complete in itself, and is not directly 
connected with the main circuit, the latter merely 
inducing in the secondary circuits the currents 
necessary for their purpose. 



CHAPTEE II 



ELECTRIC LIGHTING 

Electricity a light-producer — First exhibition of voltaic arc — In 
what it consists — Size increases with E.M.F. of current — 
Vividness — Use in lighthouses — Cause of the light — Unequal 
consumption of the carbons — Consequent unsteadiness of 
light — Means of rectification — Variety of arc lamps — Illumin- 
ative power — Electric candles — Incandescent light — Its 
principle — Difficulty originally encountered in construction of 
incandescent lamps — Obviated by improved vacuum — Descrip- 
tion of Edison's and Swan's incandescent lamps — Holders — ■ 
Generation of electric - lighting currents — Central lighting 
stations — Street illumination — Indoor illumination — Use of 
accumulators — High -potential currents necessary in main 
wires — Importance of careful insulation — English mode — 
American mode — Imperfection of American installations — 
Safety of electric lighting if properly carried out — Advantages 
of electric light — Various appliances. 

The electric spark, the electric glow, and the 
luminous discharge through rarefied gases, all made 
known (and lightning did so long before them,) that 
electricity is a light -producer. The possibility of 
using it for ordinary purposes of illumination was 



chap, ii ELECTRIC LIGHTING 223 

not understood, however, till the beginning of the 
present century, when Sir Humphry Davy first 
exhibited the " voltaic arc " in public at a meeting of 
the Eoyal Society in 1810. He employed for the 
purpose an exceedingly powerful battery of over 
2000 elements, and though the beauty and brilliancy 
of the light were beyond all question, the expense of 
its production relegated it for a considerable time 
after this to the region of experiment only. The 
arc produced by Davy, and of which so much has 
been heard since, consists of an intensely vivid band 
of light passing between two carbon pencils, which 
form part of a powerful voltaic circuit. These 
pencils, of which an 



illustration is given in 



& j 



+ 



-n. qq -ft fVk V.4- ■ FlG ' 33 " — Carbon Pencils used for the pro- 

log. OO, are frrSt DrOUgnt} duction of the arc light, the positive 

into momentary con- penc * showin3 + th ! concave f0 ™ as " 

J snmed owing to its more rapid con- 

tact and then drawn sumption. 

a short distance apart. A current of sufficient 

electro -motive force is able to overleap this gap, 

but in doing so it has to overcome a very high 

resistance, and the consequence is an outburst of 

heat and light of extraordinary intensity. The higher 

the electro - motive force, the greater may be the 

distance between the carbon points, and the more 

vivid of course will be the light. It is too vivid, in 

fact, for many purposes; it dazzles, and for the 



224 PRACTICAL APPLIANCES part iv 

interior of most public buildings, churches, hotels, 
theatres, etc., as well as for private houses, the 
incandescent mode of lighting, to be presently 
described, is far preferable. For open air illumina- 
tion, however, the arc .light presents great advan- 
tages, not the least of which is its comparative 
inexpensiveness ; and it is unrivalled in one depart- 
ment of the highest importance, viz. that of lighthouses, 
where its object is not so much to render other things 
visible as to be seen itself; it was in fact for them 
that it was first brought into use. In the search- 
lights employed for naval and military purposes, its 
great intensity renders it also specially suitable. 

The main cause of the light appears to be the 
incandescence of minute particles of carbon, which 
are carried in two incessant streams between the 
carbon pencils, the principal direction being always 
from positive to negative. On account of the great 
affinity of carbon at a high temperature for oxygen, 
combustion of the pencils takes place even when 
they are exposed to rarefied air, the positive carbon 
consuming nearly twice as fast as the negative. 1 
Consequently the distance between the two pencils is 
continually increasing, and unless means of rectification 

1 It must not be supposed, however, that combustion is the 
cause of the light, as in an ordinary oil or gas lamp, for the arc 
light can be quite satisfactorily produced in a vacuum or under 
water. 



chap, ii ELECTRIC LIGHTING 225 



are used, soon becomes too great to allow the current 
to overleap it, so that the light, after becoming 
fainter and fainter, goes out altogether. Numerous 
ingenious automatic contrivances have been devised 
to obviate this difficulty, and keep the carbons at an 
equal distance apart ; and these, combined with the 
great steadiness of current which is attainable by the 
use of the modern compound-wound dynamos, have 
almost completely done away with the flickering and 
inconstancy of the earlier arc lights. If, therefore, 
such inconveniences are suffered from in modern 
days, it is because the installations are in some 
way less perfect than they could be. 

The variety of arc lamps is very great, but their 
principle having been described, it is needless to go 
into technical details, which would be wearisome to the 
general reader. Suffice it to say that the principal aim 
of inventors is to increase the steadiness of the light by 
keeping the distance between the pencils as constant 
as possible, and by using in their construction the 
purest carbon obtainable. The illuminative power 
of an arc lamp depends of course on the size of the 
arc, and this partly on the quantity of current, so 
that for lamps which are intended to light up a large 
area, currents of many amperes are needed. 

Electric candles, which are a form of arc lamp, 
must not go unmentioned. The first was the 



226 PRACTICAL APPLIANCES part iv 

Jablockoff, so called from the name of its inventor. 
It consists of two perpendicular rods of carbon, united 
at the top by a very thin bridge of the same sub- 
stance, below which they are divided by a layer of 
some insulating material, usually plaster of Paris. 
When the current passes, the carbon bridge is rapidly 
consumed by the intense heat developed, and a 
voltaic arc takes its place, which melts the insulating 
material as the carbons gradually burn away. In 
order that their consumption may be equal, alternat- 
ing currents are used, so that the same carbon is 
alternately positive and negative. The Jablockoff 
candle is equal in illuminative power to ten, twenty, 
one hundred or more ordinary candles, according to 
its size. It was at one time in great demand, especi- 
ally in Trance, but was never found altogether 
satisfactory, and has therefore almost fallen into 
disuse. Later and better forms of electric candle 
have been devised, using air as the insulating layer, 
and with other improvements ; but they are neither 
so simple as the regulated arc lamp, nor do they give 
so high a proportion of light for the current expended 
on them, and they are therefore not much employed. 
On account of the unsteadiness and too great 
brilliancy of the arc lamps, electric lighting for 
domestic and most indoor purposes would never have 
become general had there not existed another and in 



chap, ii ELECTRIC LIGHTING 227 

this respect more satisfactory method, known as the 
incandescent. In this no gap is formed in the 
circuit, but it includes substances of very high resist- 
ance (such as carbon or platinum), which are raised 
to a white heat by the passage of the current. 
Almost every reader is acquainted with the beautiful 
little lamps known by the names of Swan and Edison, 
whose light for all ordinary purposes leaves nothing 
to be desired either in softness or brilliancy, yet 
hardly ten years have elapsed since their first ap- 
pearance. The possibility of " incandescent " electric 
lamps was indeed known long before this, but an 
insuperable difficulty seemed to lie in the way of 
their construction, for a material was needed com- 
bining two qualifications which had never been found 
united in the same substance. The first was that it 
should not, when raised to a high temperature, have 
an affinity for oxygen ; and the second, that under 
the same circumstances it should not melt. Carbon 
possessed the latter of these requirements, and plat- 
inum the former ; but no vacuum had ever been 
made sufficiently perfect to render carbon incombus- 
tible, and platinum could not of course be induced 
to raise its melting-point, already higher than that of 
any other metal. The perfecting of the mercurial 
air-pump at length rendered it possible to produce a, 
degree of exhaustion, in which parbon remained in- 

16 



228 PRACTICAL APPLIANCES part iv 

combustible for want of oxygen to combine with, and 
from that moment the future of the incandescent 
lamps was secured. Inventors by the score sprang 
into the field, each with a lamp which some special 
qualification was supposed to render superior to all 
others, but the most successful competitors were 
Swan in England and Edison in America, soon fol- 
lowed by Lane Fox in the former, and Maxim in the 
latter country. 

The principle of the incandescent lamps is the 
same, whatever differences of detail there may be in 
their construction. It consists in raising a thin 
carbon filament, enclosed in a glass vessel exhausted 
to the pitch of about the millionth of an atmosphere, 
to a state of incandescence, by the passage through it 
of an electric current. The carbon filaments are 
made of different materials by different constructors. 
Edison uses bamboo fibres, 0*04 inch in diameter and 
about 5 inches in length ; Swan selected cotton thread, 
which he soaked in sulphuric acid, thus transforming 
it into a species of parchment ; and this preparation 
is adopted by the Edison-Swan Company. Edison's 
bamboo fibres are carbonised by being placed in 
U-shaped moulds and baked in ovens. Swan's thread 
fibres, after being treated with the sulphuric acid, are 
bent into the desired form, and enclosed in hermeti- 
cally-sealed crucibles filled with coal-dust, which are 



CHAP. II 



ELECTRIC LIGHTING 



229 



subjected to the requisite heat. These carbon fila 
ments are fastened to platinum wires, 
which are fused into exhausted glass 
vessels, and their free ends connected 
by appropriate adjustments to the 
wire conveying the current. In order 
to ensure equality of resistance, the 
carbon filaments are made of exactly 
the same thickness throughout, which 
is secured by placing the carbon fila- 
ments in a hydro-carbon atmosphere, 
and passing a current through them 
till they glow. A deposit of carbon 
on the filaments is the result, the 
deposit being thickest where the 
carbons are thinnest and therefore 
hottest. This operation is called 
" flashing." At the points of junction 
with the platinum wires, however, a 
great increase of thickness is neces- 
sary in order to prevent the melting 
of the platinum by the intense heat 
developed. 1 Fig. 34 represents an 
Edison incandescent lamp. 

1 It is erroneous to suppose that the electric 
light is unaccompanied by heat. That of the voltaic arc is the most 
intense known, and we have seen that even platinum is unable to 
withstand the high temperature to which the passage of the electric 




Fig. 34.— Edison's.In- 
candescent Lamp. 
C C, carbon fila- 
ment, J J, junctions 
of the carbon fila- 
ment with platinum 
wires ; p p, plat- 
inum wires by 
means of which 
current is conveyed 
to the carbon fila- 
ment ; S E, metallic 
screw and end form- 
ing the terminals of 
the lamp, contact 
being automatically 
made with the 
similar parts of the 
socket into which 
the lamp is fitted. 



230 PRACTICAL APPLIANCES part tv 

All incandescent lamps need special holders by 
whose means the necessary connection with the con- 
ducting wire can be set up and maintained. 

The currents for electric - lighting purposes are 
generated by dynamos, which are driven by steam or 
gas-engines, or when practicable by water- power. 
For every eight or ten lamps of sixteen candle-power 
each, current is required which necessitates an ex- 
penditure of mechanical work equivalent to one indi- 
cated horse-power, so that the dynamos and engines 
must be in proportion to the amount of lighting to be 
done and the consequent current needed. 

The arc lamps used in street -lighting may be 
connected to the main circuit either in parallel or in 
series, according to the system employed. If they 
are in series, the current passes through each one in 
succession, and unless special provision is made, they 
must all be turned on or put out together. In some 
systems, however, notably the Thomson -Houston, 
which is the most popular in the United States, the 
machines are so regulated that though the lamps 
are in series, any one can be cut out of circuit if 
desired, by means of a "switch," the amount of 

current raises it. Nevertheless, on account of the small volume of 
the arc, and of the incandescent carbon filaments, they give out, 
though intensely hot themselves, a very minute amount of heat 
compared to that from gas and candle flames for the same amount 
of light. 



CHAP. II 



ELECTRIC LIGHTING 




232 PRACTICAL APPLIANCES part iv 

power consumed being in proportion to the number 
of lamps maintained. Fig. 35 shows a model electric- 
lighting station on the Thomson-Houston system. 

The circuits which supply domestic, and, in fact, 
all indoor illumination, are often, by means of trans- 
formers, or of secondary batteries, kept distinct from the 
main circuit, the latter being merely used to feed them. 
In the case of transformers this is done by induction, 
as was described in Chapter I, the apparatus being 
carefully enclosed in a locked receptacle, so that no 
inmate of the house in which it is placed can have 
access to the dangerous currents. In the case of 
secondary batteries, or accumulators, as they are very 
frequently called, the charging is done by the main 
circuit current, from which they are afterwards dis- 
connected and made to do their own work, to be re- 
connected and recharged when necessary. There are 
two objections to the use of accumulators, however — 
their cost is considerable, and they constantly require 
skilled attention. On this account, when they are 
used to supply town dwelling-houses, a number 
are placed together in a sub-lighting station, and 
each one is connected during the time the current is 
required to the house which it feeds. When it has 
to be charged, it is joined in series with the other 
accumulators belonging to the same group as itself, 
and this group is then connected to the main circuit 



chap, ii ELECTRIC LIGHTING 233 

for the requisite number of hours. 1 The advantage of 
this arrangement is that the dynamos which supply 
the main currents can be utilised to charge the 
accumulators during the daytime, and since these then 
feed their own circuits, the installation at the central 
station may be much smaller than if it had to provide 
directly all the current required during the night 
hours, as is the case when transformers are used. 
Every lamp for indoor illumination can be, and 
usually is, supplied with a separate connection to the 
conducting wire, which can be made or broken at 
pleasure, so that the lamp may be lighted or put out 
independently of any other in the same building. 
A great number also are provided with safety fuses, 
viz. a piece of wire attached to the lamp, and through 
which the current must pass. Should the latter 
attain an undue strength, the fuse melts, the circuit 
is broken, and the lamp goes out, so that all danger 
from overheating is obviated. 

In order that the distribution of electric energy 
from the central lighting station may be carried out 
as economically as possible, currents of very high 
electro-motive force (usually about 2000 volts,) are 

1 Of course accumulators cannot be charged except by con- 
tinuous-current dynamos, as the chemical decomposition necessary 
to the storage of electric energy could not be carried on with 
alternating currents, which would undo the work as fast as it was 
accomplished. 



234 PRACTICAL APPLIANCES pakt iv 

conveyed by the main wires, which it is therefore 
necessary to insulate in the most careful way. In 
England they are surrounded by layers of insulating 
material, and buried underground. In America 
they are often carried overhead, but this system is 
open to very great objection, as a high wind or heavy 
fall of snow may break down the wires, and they are 
then exceedingly dangerous, both on account of the 
high potential currents they convey, and of their 
own weight. It is to be observed, that though electric 
lighting is far more extensively used in America than 
in England and Europe generally, the technical per- 
fection of the installations is not so good. Insulation 
is not sufficiently attended to, 1 the neighbourhood of 
gas-pipes and mains to the wires is not avoided as it 
should be, so that in case of overheating of the latter, 
danger of explosion occurs ; and the system of over- 
head wires, unsightly to the last degree, gives rise to 
inconvenience and peril, which till lately met with 
less consideration than should have been the case. 
In New York, however, a regular rebellion has taken 
place against the overhead system, the wires have 
been hewn down in all parts of the city, and their 
place is being supplied as fast as possible by under- 

1 It is just to say, however, that owing to the much greater 
dryness of the climate, insulation is far more easily carried out in 
America than in England, and a system which would be dangerous 
in the latter country, might be perfectly safe in the former. 



chap, ii ELECTRIC LIGHTING 235 

ground cables. 1 So many accounts of fatal accidents 
(numbers of which are untrue,) from electric-lighting 
currents have been telegraphed across the Atlantic, 
that timid persons may well shrink from the idea of 
seeing them come into general use in England. There 
is no real occasion for alarm, however. If the con- 
ducting wires are thoroughly insulated, so as to 
render the touching and handling of them impossible, 
very carefully joined where joins are necessary, and 
thick enough not to be overheated by the strongest 
current they would ever be called upon to convey, no 
possible danger can occur. If these precautions are 
neglected, the consequences may be disastrous, 
though not more so than if due care is not taken 
with those equally perilous agents, gas and steam. 
An electric light installation set up by competent 
electrical engineers, allowed to provide all those safe- 
guards which they know to be necessary, is perfectly 
harmless ; but if cheapness is made the first desider- 
atum, and bad workmen and bad materials are em- 
ployed, the result is as unsatisfactory and dangerous, 
as a similar mode of proceeding would be, if followed 
by gas and railway companies. 

The advantages of the electric over other kinds of 
artificial light are in many respects very great. In 
the first place, it is much healthier, for it does not, as 

1 January 1890. 



236 PRACTICAL APPLIANCES part iv 

they do, take up oxygen and give out carbonic acid 
gas. Then, it does not perceptibly raise the temper- 
ature of an enclosed space, so that a room or building 
lighted by it does not become rilled with overheated 
air. Again, owing to its near approach to the com- 
positibn of sunlight, colours are clearly distinguishable 
by it, which are confused with one another or altered 
in hue by gas or candle light ; and for the same reason, 
photographic and other chemical work can be carried 
on by its means, when they would otherwise be im- 
possible. The electric light is, in fact, particularly 
rich in chemical rays ; and the arc light, owing to 
the preponderance of violet in its spectrum^ gives a 
cold and rather ghastly appearance to persons and 
things. This is not the case with the incandescent 
lamps ; and, in fact, when sunlight and any kind of 
electric light are seen at the same time, the latter is 
found to have by comparison a reddish hue. The 
weird effect of the arc light is probably in great part 
due to our bein£ accustomed to the very red tint of 
gas ; and, of course, where the two illuminants are 
seen close together, as frequently happens in London, 
the result is exceedingly unpleasant. 

Besides its extensive adoption on land, and for 
lightnouses, where its superiority over any other kind 
of illuminant is incalculable, the electric light is also 
much used now on board ship, as well for ordin- 



chap, ii ELECTRIC LIGHTING 237 

ary purposes as for signalling ; and it is of interest to 
note that by its means the Suez Canal has been 
rendered navigable by night, its passage during the 
hours of darkness being permitted to all vessels 
carrying electric search-lights, arranged according to 
the regulations laid down by the Canal Company. 
The use of electric search-lights on board warships 
has already been referred to, and their importance 
can hardly be overestimated, now it is acknowledged 
that nets are of little if any protection against tor- 
pedoes, so that the only safety from these formidable 
engines of destruction, lies in being aware of their 
approach. This can only be ensured at night, by 
throwing powerful streams of light in every direction 
from which danger is to be apprehended, and the 
intensity of the arc light makes it specially suitable 
for this purpose, rendering the detection of torpedo- 
boats a matter of comparative ease. 

Among the minor uses to which the electric light 
has been put, the production of very beautiful scenic 
effects in theatres may be mentioned; and it has even 
been made to contribute to the personal adornment 
of the actors. " Electric hairpins " are described in 
Electricity in the Service of Man} They are simply 
miniature glow lamps arranged so as to simulate 
gems of various colours, and supplied with the neces- 

1 P. 562. 



238 PRACTICAL APPLIANCES pakt iv 

sary current by a tiny battery enclosed in a gutta- 
percha box, small enough to be concealed in the hair 
or head-dress. Another very ingenious contrivance 
is mentioned in the same work, and consists of an 
apparatus enabling a faint light like that of the Will- 
o'-the-wisp to play about the heads of those wearing 
it. The frequent variations of intensity which are 
necessary in theatrical lights render indispensable, 
contrivances enabling any particular lamp, or whole 
series of lamps, to be put in and out of circuit as 
desired ; and, in fact, such arrangements are required 
in all indoor illumination, as the number of lamps in 
use in any building is constantly altering. If many 
lamps are put out of circuit at the same time, it is 
necessary, unless compound - wound, i.e. constant 
potential - difference, dynamos are supplying the 
current, to increase the resistance encountered, 
as the current would otherwise be too powerful 
for the remaining lamps. To accomplish this, 
arrangements are made by which resistance-boards 
or coils can be included or cut out of circuit at 
will. The latter are merely wire coils usually made 
of German silver ; the former consist of a number 
of parallel carbon rods fixed on a wooden board, any 
or all of which can be included in circuit as desired. 
A very curious appliance of the electric light is 
mentioned in The Electrician for 10th January 1890 



chap. Ti ELECTRIC LIGHTING 239 

viz. its being used to lure fishes and submarine 
animals into a trap employed in the deep-sea investi- 
gations undertaken by the Prince of Monaco. The 
light and the battery supplying it were placed in a 
wire trap, and in order to keep the box in which the 
battery was enclosed from being crushed by the 
enormous pressure of water, it was connected with 
an air balloon in such a way that air could pass from 
one to the other. As the pressure on the balloon 
increased in its descent through the water, air was 
forced from it into the box, thus providing the in- 
terior of the latter with an equal and contrary 
pressure to that exerted on its exterior, the balloon 
itself gradually diminishing in size till it was reduced 
to a fraction of its original volume, and expanding 
again when the apparatus was raised and the pressure 
of water decreased. The experiment was so success- 
ful, that the Prince hopes on his next expedition to 
obtain photographs of the ocean bed by means of the 
electric light. 

For purposes of electric lighting and transmission 
of power, it is obviously necessary that both supplier 
and consumer should be able to gauge, with fair 
exactitude, what amount of electric energy is used ; 
but the measurement cannot be accomplished in so 
simple and certain a way as the measurement of 
gas consumption for instance. The latter depends 



240 PRACTICAL APPLIANCES part iv 

upon quantity only, but the measurement of electrical 
energy depends upon the electro -motive force (or 
pressure), as well as on the strength of current. If 
the former were to fall below its right value, the 
consumer would not be getting his due proportion of 
energy, though the current strength might be fully 
maintained. The meter employed, therefore, ought 
to be a volt-meter or measurer of electro-motive force, 
as well as an ampere or am -meter, measurer of 
current strength. It is not easy to combine these 
two requisites in a simple and efficient form, and 
many inventors have employed their ingenuity in 
this direction, without as yet obtaining a perfectly 
satisfactory result. The best appears to be given by 
the clock -meter of Professors Ayfton and Perry, 
whose most important constituent is a good clock, 
the works of which are electrically connected to the 
wires conveying current to and from the house, in 
such a way as to be affected both by its pressure and 
strength. The clock consequently loses in exact 
proportion to the amount of energy consumed. If, 
however, a constant electric pressure between the 
house mains can be secured, it is sufficient to 
measure only the quantity of current that passes. 



CHAPTEE III 

TRANSMISSION OF POWER BY ELECTRICITY 

Definition of power — Units of power — Various agents for the trans- 
mission of power — Its importance — Loss incurred — Advantages 
of electricity as a transmitter of power — Its probable universal 
adoption — Requisites for the transmission of power by elec- 
tricity — Electric railways — Telpherage — Attempts to utilise 
electricity for road traffic — Other applications — Electric 
launches. 

By the term " power " engineers understand the 
rate of doing work. The transmission of power 
therefore means not simply the transmission of 
work, but of a given amount of work in a given 
time. The ordinary unit of work in England is the 
foot 'pound, viz. the amount of work done in raising 
one pound to the height of one foot. For engineer- 
ing purposes horse-power, which is equivalent to 
33,000 foot pounds per minute,is often used as the unit 
of power; and in electrical engineering the unit of 
power is the Watt, which equals 44J foot pounds 



242 PRACTICAL APPLIANCES part iv 

per minute, so that 746 Watts are equivalent to one 
horse-power. 1 

Power may be transmitted by various agents. 
We may pull a handle connected to a bell wire, and 
thus ring a bell in a distant room ; the wire is here the 
transmitter of power. Or there may be no bell wire, 
but by pulling the handle w r e may cause the com- 
pression or rarefaction of air in a tube, at the other 
end of which the bell will ring. In this case the trans- 
mitter of power is the air. Or the bell may be an 
" electric bell,'' and by the pressure of our hand on a 
button in the wall we may start an electric current, 
by means of which the ringing is accomplished; 
electricity then being the transmitter of power. It 
is the transmitter only, however, not the source. 
That, in each of the three cases mentioned, is the 
human hand, whose action in pulling or pressing the 
bell handle, originates the power transmitted by the 
wire, the air, or the electric current. 

Even from such a trivial example as the ringing 
of a bell, it is easy to see what importance attaches 
to the transmission of power, and what saving of 
time and labour it may enable us to effect. Because 
the mechanical action of pulling or pressing a bell 



1 Electrically defined, the Watt is the power developed in a 
circuit through which a current of one ampere is flowing, and 
which has a potential difference of one volt between its ends. 



chap, in TRANSMISSION OF POWER 243 

handle, can be transmitted to a distance, a slight 
exertion and a small bell will suffice ; but if we could 
only ring the bell at the spot where we ourselves 
might be, a much larger bell and far greater exertion 
would be necessary, in order to attract the attention 
of those at a distance. 

In all transmission of power some loss occurs. 
We never get back exactly the amount of power 
originally expended, and where the distance is great 
the loss is often very considerable. It is less where 
electricity is employed, than in any other case, how- 
ever, and consequently now that dynamos and elec- 
tro-motors are so much less costly of construction 
than they were a few years ago, electricity bids fair 
to become the cheapest, as well as the most rapid, 
cleanest, and most easily controlled transmitter of 
power known. It has not, indeed, yet been applied 
to great distances and large power; but whatever 
initial difficulties there may be in thus utilising it, 
they are not of such a formidable nature as to prevent, 
or even long retard, its widespread adoption ; and 
not scientists and electrical engineers only, but the 
foremost statesman in England * has ventured to 
look forward to the time when noisy and crowded 
factories will be abolished, and each workman sup- 

1 Lord Salisbury in his speech at the First Dinner of the Institute 
of Electrical Engineers. 

11 



244 ELECTRICAL APPLIANCES part iv 

plied at Lis own house with the power necessary for 
his work, distributed from a central station as elec- 
tric-lighting currents now are. In fact, with very 
little increase of cost and material, the same station 
would serve both purposes. The currents are not 
wanted for lighting during the daytime ; and though, 
where accumulators are used, they must be charged, 
this would not occupy all the dynamos at a central 
station, nor any of them during a whole day, so that, 
at no very distant future, we may contemplate the 
establishment of centres which shall supply each 
household with light during the hours of darkness, 
and with power during the day. This system is 
already extensively adopted in America, to supply 
workshops where only small power is required, 
such as those of tailors, shoemakers, watchmakers, 
etc., but it has not yet been attempted on a large 
scale. 

The requisites for electrical transmission of power 
are — 

1. Mechanical power of some kind to drive the 
dynamos. 1 

2. The dynamos or current-generators themselves. 

1 It is not intended to state that the transmission of power by 
electricity, can only take place by means of currents generated by 
dynamos. Batteries may be used ; but for reasons already pointed 
out they are unsuitable, except when (as in telegraphy) the power 
to be transmitted is very small. 



chap, in TRANSMISSION OF POWER 245 

3. An electro-motor or motors, placed where the 
supply of power is required. 

4. An electrical connection between the dynamos 
and the electro-motors, unless accumulators are used, 
when such connection is of course unnecessary, and 
the dynamos are employed for charging. 

The driving of the dynamos is generally accom- 
plished either by water-power or steam. Wherever the 
former is available, it is the best and most economical 
to use, and the continual regret of electrical engineers 
is the amount of energy running to w 7 aste in our 
streams and rivers, their dream for the future being 
that as electrical transmission of power becomes 
better understood, and more easily available over long 
distances, the water-power of every country may be 
found sufficient for its needs. Dynamos and electro- 
motors have been described in a previous chapter, 
and with regard to the connection between the two, 
it varies according to the nature of the work to be 
performed, and with other local and technical causes. 

The most important use to which electricity as a 
motive-power has been put, is that of traction. Elec- 
tric railways, or, more correctly speaking, tramways, 
as they are used only over short distances and for 
light traffic, are becoming well known, though not yet 
common. The motors are usually placed under the 
floor of the cars, and are supplied with current either 



246 



ELECTRICAL APPLLANCES 



PART IV 



through the rails on which the cars run, or by separate 
underground conductors, or by overhead conductors 
as in Fig. 36 ; or they may be driven by accumulators, 
which in some ways simplifies matters, as the rails 

A 




1~T 







Ml V=f^ 



ULJU 



Fig. 36. — Electric Tramcar supplied with current by overhead conductors. 

need not then be insulated, and each car can be inde- 
pendent of its fellows. In other respects this system 
has disadvantages; the weight of the accumulators, 
which of course must be carried by the cars, being one, 
and the double transformation of energy they entail 
being another, as more waste is thereby necessitated. 1 

1 Where accumulators are employed, mechanical energy (that 
used to turn the dynamo armatures,) must first be converted into 
electrical energy, then the latter changed in the accumulator to 
stored-up chemical energy, which on being liberated is transformed 
once more into electrical energy, and then through the electro- 



chap, in TRANSMISSION OF POWER 247 

The telpherage system, formerly used for the 
transport of goods only, but now beginning to be 
employed for passengers also, is specially suitable 
and economical for hilly country, and was introduced 
by Professors Ayrton and Perry, and the late Professor 
Fleeming Jenkin. In this system the traffic is 
carried on overhead, the conductors and trucks being 
suspended from poles erected about fifty yards apart. 
The conductors sometimes consist of two steel cables 
fixed one above the other, the trucks being suspended 
between them, and the upper cable bearing most of 
the weight. More usually, however, there is only 
one steel cable (as in the passenger telpher line at 
the Edinburgh Exhibition,) by which the wdiole 
weight is supported. The trucks are provided with 
wheels through which, as they run along the con- 
ductors, the necessary current is supplied. 

Attempts have been made to utilise electricity for 

ordinary street traffic, as w r ell as for railways. In 

this case the motors which propel the vehicles are 

of course driven by accumulators, and success has 

been attained for short distances and light weights ; l 

motor back into mechanical energy. Where the dynamos supply 
the motors directly, two transformations only take place, from 
mechanical into electrical energy in the dynamo, and from electri- 
cal into mechanical in the motor. 

1 An electric dogcart, made for the Sultan of Turkey in 1888, 
gave its imperial owner so much satisfaction that he has since 
ordered another. 



248 ELECTRICAL APPLIANCES part iv 

but for long distances, difficulties occur through the 
necessity of exchanging the exhausted accumulators 
for fresh ones ; and when the cart or carriage is 
heavy, it is impossible to stop it with the rapidity 
often necessary in crowded streets. This difficulty 
arises from the increase of weight occasioned by the 
use of large accumulators, which gives too great a 
momentum to the vehicle. 

Electricity has been made useful for locomotion 
in mines, for boring rocks, for lifts, for cranes, for 
brakes, (the Westinghouse brake being familiar, at 
least by name, to all readers,) and for driving 
machinery in factories. 

Electrically-propelled ships have not been at- 
tempted on a large scale, but many electric launches 
are made, and found to answer exceedingly well. 
Their chief inconvenience lies in the fact that owing 
to their motors being driven by accumulators, they 
cannot go far from the charging station, unless 
indeed (as has been done on the Thames,) sub- 
stations are placed at intervals on the line of 
route, so that exhausted accumulators can be re- 
placed by fresh ones when necessary. The most 
recent application of electricity to naval purposes, 
has been independently made by the French and 
Belgians, in submarine electric launches or torpedo 
boats, recalling on a small scale the marvellous 



chap, in TRANSMISSION OF POWER 249 

vessel in Jules Verne's Tiventy Thousand Leagues 
under the Sea. 

The various uses to which electricity as a motive 
power has been put, have now been enumerated, but 
there is one to which neither it nor any of its rivals 
has yet been successfully applied, but which 
nevertheless we can hardly help regarding as probably 
one of its important functions in the future — this is 
aerial navigation. If our descendants are to travel 
in balloons, it will certainly be through the agency 
of electricity ; and as the various electrical engines 
become more and more perfected, we can hardly 
doubt that in time a sufficiently light means of 
storing electric energy will be devised to make that 
dream of past nges — a flying machine — an actual 
reality. 



CHAPTEE IV 

THE ELECTRIC TELEGRAPH 

Early suggestions for using electricity and magnetism as a means 
of communication — Yolta's and (Ersted's discoveries the 
foundation of modern telegraphy — Number of inventors who 
have contributed to its perfection — Wheatstone and Cooke's 
telegraph — Discovery of the " earth -circuit " — Introduction 
of the "translation" system — Recording and non-recording 
instruments — Morse's services to telegraphy — His embosser and 
ink-writer- — The Morse key — Speed of transmission of messages 
— Hughes printing telegraph — The Morse sounder — Submarine 
signalling — The syphon recorder — Manner of conveying and 
insulating overland wires — Submarine cables — Duplex and 
quadruplex telegraphy — Telegraphic currents generated by 
batteries. 

It seems that the first really practical suggestion for 
using electricity as a means of communication, was 
made by the anonymous author of a letter to the 
Scots Magazine in 1753. The suggested apparatus, 
though cumbrous and complicated according to modern 
ideas, was very ingenious, but there is no record of its 
having been actually used. The writer, and such of 
his readers as took any interest in the subject, were 



chap, iv THE ELECTRIC TELEGRAPH 251 

doubtless of opinion that at any rate the idea was an 
entirely novel one, but in this they were to some 
extent mistaken. Though nothing was known about 
" current " electricity until the end of the eighteenth 
century, there had been vague ideas long before then, 
that magnetism might be in some way utilised for 
purposes of communication. About 1750 a certain 
Joseph Glanvil, rector of Bath, who we are informed 
was a "learned writer upon abstruse and mystical 
subjects," wrote as follows in a treatise entitled " The 
Vanity of Dogmatising," and in which he speaks of 
" supposed impossibilities which may not be so." 

" That men should confer at very distant removes 
by an extemporary intercourse is a reputed impossi- 
bility ; but there are yet some hints in natural 
operations that give us probability that 'tis feasible, 
and may be compassed without unwarrantable 
assistance from demoniack correspondence. That a 
couple of needles equally touched by the same magnet, 
being set in two dials exactly proportioned to each 
other, and circumscribed by the letters of the alphabet, 
may effect this ' magnale ' (i.e. important result), 
hath considerable authorities to avouch it. * The 
manner of it is thus represented. Let the friends 
that would communicate take each a dial, and having 
appointed a time for their sympathetic conference,- let 
one move his impregnate needle to any letter in the 



252 ELECTRICAL APPLIANCES part iv 

alphabet, and its affected fellow will precisely respect 
the same. So that would I know what my friend 
would acquaint me with, 'tis but observing the letters 
that are pointed at by my needle, and in their order 
transcribing them from their sympathised index as its 
motion directs, and I may be assured that my friend 
described the same with his, and that the words on 
my paper are of his inditing. Now though there 
will be some ill contrivance in a circumstance of this 
invention, in that the thus impregnate needles will 
not move to, but avert from each other (as ingenious 
Dr. Browne hath observed), yet this cannot prejudice 
the main design of this way of secret conveyance ; 
since it is but treading counter to the magnetic 
informer, and noting the letter which is most distant 
in the abecederian circle, from that which the needle 
turns to, and the case is not altered. Now, though 
this desirable effect possibly may not yet answer the 
expectations of inquisitive experiment, yet 'tis no 
despicable item, that by some other such way of 
magnetic efficiency it may hereafter with success be 
attempted, when magical history shall be enlarged by 
riper inspections ; and 'tis not unlikely but that present 
discoveries might be improved to the performance." 1 

1 The above curious passage is quoted from a letter by the Rev. 
Canon Jackson of Leigh-de-la-Mere, Chippenham, the well-known 
antiquarian, to the Bath Chronicle in October 1890. 



chap, iv THE ELECTRIC TELEGRAPH 253 

Could Joseph Glanvil's spirit revisit us now, he 
would see his expectations more than fulfilled, though it 
has not been by " riper inspections of magical history," 
but of natural science that the "reputed impossi- 
bility " has become possible. Forty years after the 
treatise "On the Vanity of Dogmatising" saw the light 
Volta had made his first battery, and twenty years 
later still, came the announcement of (Ersted's famous 
experiments on the action of electric currents on the 
magnetic needle. These two discoveries are the 
foundation-stones of our modern system of telegraphy, 
which would be impossible without a steady current, 
as distinguished from the momentary rush of a 
discharge, and to which the deflections of the magnetic 
needle, under the influence of electric currents, con- 
tributed a code of signals far superior to any which 
had previously been attempted with electricity. 

So many scientific inventors have aided in bringing 
the electric telegraph to its present degree of 
perfection, that it w r ould be impossible in the short 
compass of a chapter of the present volume, even to 
name them all. The first really workable telegraph 
introduced into England, was that of Wheatstone and 
Cooke, who at first employed five needles and as many 
wires to transmit their signals, but soon finding the 
cost and inconvenience of this method rendered its 
general adoption impracticable, they reduced the 



254 ELECTRICAL APPLIANCES paet iv 

number of needles to two and finally to one, whose 
deflections to the right and left were so combined, as 
to represent all the letters of the alphabet. In all the 
earlier telegraphic circuits it was considered necessary 
to have a return wire, but in 1838 Steinheil discovered 
that this could be dispensed with, and that if the two 
ends of the conducting wire were connected to earth, 
one at the sending the other at the receiving station, 
the earth itself would play the part of the return wire. 
This discovery of the " earth circuit " was of the 
greatest practical importance, owing to the reduction 
in cost which it brought about. 

Another very important improvement in the 
construction of electric telegraphs, is the system of 
" translation " which was introduced by an English- 
man, Edward Davy. The principle consists simply 
in cutting up one long circuit into a number of short 
ones, which can be automatically connected whenever 
a signal is transmitted. The advantage of this system 
is that even on a long circuit a weak current may be 
employed, as it will only have a comparatively short 
distance to traverse before its work is taken up by 
another. 

The telegraphic instruments in modern use may 
be divided into two classes, recording and non- 
recording. To the former (whose signals are merely 
momentary and leave no after traces), belong the 



chap, iv THE ELECTRIC TELEGRAPH 255 

needle and dial telegraphs and the sounders. To the 
latter belong the embossers, ink- writers, and type- 
writers. In international telegraphy and in the 
English Post- Office, Morse's instruments, of which 
some are recording and some non- recording, are 
almost exclusively used, and a slight description of 
them will therefore be given. Their inventor, Morse, 
who received considerable sums of money from 
various nations, in recognition of his services to inter- 
national telegraphy, was an American, and his first 
experiments in the direction in which he afterwards 
became so famous, were made as early as 1834, when 
he had already twice visited Europe. He laboured 
at first under the disadvantage of very inadequate 
electrical knowledge, and indeed his mind was origin- 
ally turned to what became his life-work, by the 
accidental acquaintance made on one of his return 
vova^es to America, with a fellow-traveller. Professor 
Jackson of Boston, who was en^a^ed on electrical 
experiments. At a later period Morse w T as indebted 
to his subsequent partner, Leonard Gale, for various 
suggestions as to the chemical part of his work. 

Morse's embossing instrument leaves a permanent 
record of the message sent, by indentations on a slip 
of paper. These are made by means of a small 
electro-magnet with a movable armature, to which is 
attached a hard pointed piece of metal. Whenever a 



256 ELECTRICAL APPLIANCES part iv 

current passes through the instrument the armature 
and stylus are drawn forward, and the latter brought 
into contact with a slip of paper, unrolled and passed 
on by clockwork. When the current is interrupted, 
the armature and stylus return to their former position, 
to be again drawn forward as soon as a current is set 
going. In this way a series of scratches is made on 
the paper, the marks being longer or shorter accord- 
ing to the length of contact, and divided by spaces 
varying with the time of interruption of the current. 
The short marks are known as dots, and the longer 
ones as dashes, and by suitable combinations of them 
all the letters of the alphabet, the numerals, punctua- 
tions, and various signs are expressed in a system of 
signals, known as the Morse code, which is now 
almost universally adopted in Europe and America. 1 
Morse's ink-writer, which is in most general use, is 
on the same plan as the embosser, but the place of 
the stylus is taken by a small disc kept constantly 
wetted with ink, by means of a suitable mechanical 
arrangement, so that ink-marks instead of scratches 
are impressed on the paper. 

The incessant interruptions of current necessary 
in the Morse, and in all telegraphic instruments, are 
brought about by means of a commutator or key, by 

1 The Morse code is used in the needle telegraphs also, an inflec- 
tion to the left representing a dot, and one to the right a dash. 



CHAP. IV 



THE ELECTRIC TELEGRAPH 



257 



which the operator closes and opens circuit at will. 
The Morse key is simply a brass lever kept in posi- 
tion by a spring S, as shown in Fig. 37. When the 



Battery Wire 




Relay Wire 



[Line Wire 
Fig, 37.— The Morse Key. 

operator presses down the button B, he causes c d 
to come into contact, and by their means sends a 
current from the battery into the line wire. When 
B is not pressed down, e f are in contact as in the 
figure, and a current coming along the line passes 
round the relay electro -magnet (see Kg. 38) and 
works the relay tongue (a). This closes the circuit of 
a local battery, which sends a sufficiently strong current 
round the electro -magnets of the receiving instru- 
nient to pull down the lever of the embosser or ink- 
writer, an operation requiring too much force to be 
accomplished by the weak current which comes 
along the line. Polarised relays, i.e. relays in which 
the core of the electro-magnet is formed of a steel 
magnet, whose poles are not reversed by change of 
direction in the current, are almost always used. 



2 5 8 



ELECTRICAL APPLIANCES 



PART IV 



It is evident that the speed of transmission of 
messages must depend to a great extent on the 
number of currents required, to form the different 

S 




Local Battery 



Fig. 38. — Polarised Relay. S N, permanent steel magnet whose north-seeking 
pole is bifurcated, its branches making the core of the electro -magnet 
M M, and terminating in the pieces n and n', between which works the 
soft -iron armature or tongue a; connection is made with the local 
battery by means of the contact-piece C whenever a current passes 
through the coils M M. 

letters. In the Morse system about three currents 
are wanted on an average for each letter. Inventors 
have therefore endeavoured to construct apparatus 
in which fewer should be necessary. The most 
successful of these is Hughes' printing telegraph, 
which requires only one current for each letter, and 
which can also print its messages (transmitted in a 
third of the time occupied by a Morse instrument), 
in Roman characters. The apparatus is, however, 



chap, iv THE ELECTRIC TELEGRAPH 259 

so complicated that it can only be entrusted to a 
thoroughly trained and skilled operator ; its use is 
therefore confined to very important telegraphic lines. 

The Morse sounder (for which the Morse key is 
used as a commutator,) is a non-recording instru- 
ment, and its signals appeal to the ear instead of to 
the eye. Its simplest form is one in which a small 
electro-magnet is supplied with an armature in the 
form of a lever, the free end of which works between 
two stops, and taps against one when the current 
starts, and against the other when it ceases. The 
intervals between the taps are made longer or shorter 
according to whether a dash or a dot is to be repre- 
sented, a short pause corresponding to a dot and a 
longer one to a dash. It is evident that in this 
system mistakes are more liable to be made in a 
message than where reference can be had to recorded 
signs. Nevertheless, with skilled operators errors 
very rarely occur, and the greater rapidity with 
which transcription can take place offers an advan- 
tage which in America, at all events, outweighs other 
considerations, for the sounder is used almost to the 
exclusion of any other instrument. It is also widely 
adopted in India. 

Submarine signalling, where the distance is great, 
requires special and very delicate instruments, the 
currents being too weak to work satisfactorily with 

18 



260 ELECTRICAL APPLIANCES paet iv 

any ordinary apparatus. 1 For a considerable time 
Sir William Thomson's mirror galvanometer (described 
in Chapter iii of Part III, p. 174,) was almost ex- 
clusively used, but owing to the great fatigue to the 
eyes incurred by watching the movements of the 
little spot of light for a long time together, this 
instrument has now been to a great extent superseded 
for signalling purposes, by another invention from 
the same high quarter, the syphon recorder, which 
by means of a flat coil of wire in circuit with the 
line, and suspended between two powerful electro- 
magnets, communicates motions to a very fine glass 
syphon, dipping into an insulated metal ink-vessel, 
and having its other end adjusted over a paper strip. 
When a message is to be recorded the vessel of ink 
is connected with a charged conductor, and the result 
is to force the drops of ink on to the paper, unrolled 
by clockwork, as in the Morse instruments. The 
mechanism is so arranged that when a current is 
passing, the point of the syphon moves alternately 
to one side and the other of the centre of the paper, 
and thus two lines of ink-spots are made, one of 
which corresponds to the dots, and the other to the 
dashes of the Morse code. 

1 The current given out by the Atlantic cable, when twenty-five 
words per minute are being transmitted, is one-millionth of an 
ampere. 



chap, iv THE ELECTRIC TELEGRAPH 261 



It is hardly necessary to describe the mode in 
which ordinary overland telegraph wires are conveyed 
from place to place. We are but too familiar with 
the gaunt wooden posts, and the wire lines suspended 
from them which make our railroads even more 
hideous than they would otherwise be. The wires 
thus employed are made of galvanised iron (iron 
coated with zinc,) and every post carries insulating 
supports through which the wires pass. In 
England, and Europe generally, overhead wires are 
replaced in large towns by an underground system, 
in which a number of wires are enclosed in iron 
or earthenware pipes and buried in the soil. 
These wires are made of copper, and each one is 
insulated by a gutta-percha covering. But for the 
greater cost and small mechanical strength of copper 
wires, they would always be preferable to iron for 
telegraphic purposes, owing to their much lower 
specific resistance, and their greater power of with- 
standing exposure to the weather. If the wares 
break, come into contact with other wires, or with 
conductors connected to the earth, there ensue total 
or partial interruptions of the currents they convey, 
technically known as " faults." Various means are 
taken to prevent these and to ensure their speedy 
remedy when they do occur. The most important 
precautions are the thorough insulation of the wires. 



262 ELECTRICAL APPLIANCES part iv 

the frequent introduction of " testing-boxes " along 
the circuit, which enable the real position of the fault 
to be speedily discovered, and (in the case of over- 
head wires,) the metallic connection of each one of 
the insulating supports with the earth. This obviates 
the danger of one wire leaking into another (which 
might otherwise frequently occur in wet weather), by 
supplying an easier and more direct path to the 
earth for the leaking current. These earth connec- 
tions of the insulators also serve the important 
purpose of lightning conductors, which are indis- 
pensable both along the line and at the signal 
stations, as without them the lightning discharge 
might travel along the wires, not only interrupting 
the messages, but destroying the telegraphic instru- 
ments, and injuring or even killing the operators. 
Such accidents have more than once happened. In 
wires carried underground, another difficulty has to be 
contended against, viz. the taking up by the wires 
of a static charge of opposite sign to that of the 
surrounding earth, which in fact acts as one coating, 
while the wires act as the other, of a Leyden jar. 
This condition of things weakens the current and 
prevents rapid signalling ; but it is not often that 
underground wires are of sufficient length for the 
currents they convey to suffer seriously from this 
cause. It is far otherwise with the submarine cables, 



CHAP. IV 



THE ELECTRIC TELEGRAPH 



263 



some of which (as those which cross the Atlantic,) 
are 2000 miles or more long. Special means have then 
to be taken to obviate the delay, and even failure 
of transmission. The most effectual 
seems to be employing alternate 
positive and negative currents, as 
then each one as it passes through 
the cable counteracts the effect of 
its predecessor. The use of the 
exceedingly delicate recording instru- 
ments already referred to, is another 
important consideration, as a much 
weaker current is then able to suflice. 
Submarine cables, especially when 
of great length, require to be exceed- 
ingly strong, in order to withstand Fio. 30.— Section of an 

Atlantic Cable. K, 

the enormous pressure of water they 




encounter when being lowered, the 

buffeting of the w T aves near shore, 

and the friction against the uneven 

and often rocky bed of the ocean. 

The Atlantic cables, of which there 

are now nine, are made of seven 

strands of copper wire, covered 

with four layers of gutta-percha encased in tanned 

hemp. Over this are twisted stout iron w T ires 

also covered with hemp. Fig. 39 gives a 



outer casing of tanned 
hemp ropea enclosing 

stout iron wires ; II, 
inner casing of tanned 
hemp ropes surround- 
ing the four gutta- 
pereha layers g g; 
C C, strand of copper 
wire (untwisted), 
forming the con- 
ductor by means of 
which the messages 
are transmitted. 



264 ELECTRICAL APPLIANCES part iv 

representation of a portion of an Atlantic cable 
showing the different layers. 

One of the most important improvements in 
modern telegraphy is the introduction of the duplex 
system, which enables two messages to be sent in 
opposite directions along the same wire at the same 
time. In order to do this, it is necessary that when a 
signal is transmitted from one station to another, the 
receiving instrument at the sending station should 
not be affected, but remain free to indicate any 
message travelling to it from the opposite direction. 
This end is attained in various ways, and not only 
has duplex telegraphy come into constant use, but 
still further developments have taken place, so that 
a single wire is now able to convey four or more 
messages at the same time. Quadruplex telegraphy, 
however, entails the necessity for two sending and 
two receiving instruments at each station. 

Currents of low electro-motive force are the most 
suitable in telegraphy, as they are then less liable 
to leak, and the mechanical work required of 
them is very slight. They are consequently 
almost exclusively generated by galvanic batteries. 
A form of Daniell's cell has been chiefly adopted 
in England, but the Leclanche is also excellent 
for telegraphic purposes and is coming widely 
into use. 



chap, iv THE ELECTRIC TELEGRAPH 265 

This slight and inadequate sketch can give but a 
faint idea of the enormous proportions to which the 
science of telegraphy, for it is a science in itself, has 
now attained, but indeed no assertion of the fact is 
necessary, for we are brought face to face with it 
every day and almost every hour of our lives. 
Those multitudinous wires spreading over our con- 
tinents and spanning our widest oceans, may well 
strike us with a feeling somewhat akin to awe, 
for they are the nerves of the world, connecting its 
centres of intellectual, political, and commercial 
activity with the remotest individual life, and Hash- 
ing interchanges of thought between nation and 
nation. 



CHAPTEE V 

THE TELEPHONE 

Novelty of the idea of transmitting spoken words— Reis's telephone 
— Graham Bell's telephone — Weakening of the sound in trans- 
mission — Invention of the microphone — Its principle — Com- 
bination with the telephone — Transmitting and receiving instru- 
ments used in England — Feebleness of telephone currents — 
The Telephone Exchange system — Transmission of concerted 
music— Telephone wires — Necessity of guarding against induc- 
tion — Means employed — Effect of earth currents and atmo- 
spheric electrical disturbances — Sensitiveness of telephone 
made useful as a test for weak currents — The microphone 
employed for medical purposes — The phonograph. 

As we have seen in the preceding chapter, the possi- 
bility of communicating at a distance by means of 
magnetic signs, though it had never been successfully 
attempted when the electric telegraph was invented, 
was no new idea. The transmission of spoken words, 
however, had not even been thought of, and its 
accomplishment came with almost as much surprise 
on scientific men as on the general public ; but 
during the last twenty-five years we have become so 



chap, v THE TELEPHONE 267 

used to talking with persons many miles away from 
us, that the thing has .almost ceased to excite wonder, 
and in many of our large towns telephonic communi- 
cation is becoming as important and common as that 
by telegraph. 

The first telephone was invented by a German, 
Reis, in 18G0. It could not, however, transmit 
articulate speech, but only musical notes, whose 
vibrations are so much less complicated than those 
of the human voice in speaking, that they are far 
more easily reproduced. 1 One of the earliest forms 
of Iieis's telephone consisted of a conical tube of 
wood, across the narrow opening of which was 
stretched an exceedingly fine membrane. One end 
of a narrow strip of platinum foil rested on the centre 
of this membrane, while the other was attached to 
a binding screw. A second strip of platinum 
attached to another binding screw, and having a 
small pointed projection for making contact, was so 
adjusted, that one end just touched that portion of 
the first platinum strip which rested on the mem- 
brane. The binding screws were connected to a 
battery, and to a wire though which the sounds were 

1 The possibility of reproducing sounds at a distance, depends 
on the fact that each one is caused by certain definite and periodic 
vibrations. If these can be transmitted in their entirety to 
another place, the sound must naturally be reproduced at that 
place. 



268 ELECTRICAL APPLIANCES part iv 

to be transmitted. When a musical note was 
sounded close to the membrane, the latter was set in 
oscillation, and at each complete vibration (i.e. move- 
ment to and fro), made and broke circuit once, by 
means of the motion imparted to the platinum strips. 
This interrupted current was transmitted through 
the line wire to the receiving instrument, which con- 
sisted of a violin, on whose bridge was fastened an 
upright knitting-needle, enclosed in a coil of fine 
silk-covered copper wire. The alternate magnetisa- 
tion and demagnetisation of the knitting-needle as 
the current flowed or ceased in the coil, produced 
the sound which always occurs when iron is thus 
treated; but since the number of times the needle 
was magnetised and demagnetised, exactly corre- 
sponded to the number of vibrations communicated 
by the musical note to the membrane whose motions 
started and interrupted the current, those vibrations 
were again exactly reproduced in the motion trans- 
mitted by the needle to the air, and the note was 
sounded once more. Eeis afterwards made many 
improvements in his telephone, but he never suc- 
ceeded in getting it to speak. In the first place, 
no instrument depending entirely on interrupted 
currents would be delicate enough for this purpose, 1 

1 The experiments of Graham Bell, whose telephone is described 
in the text, led him to distinguish between three kinds of currents, 



CHAP. V 



THE TELEPHONE 



269 



and in the second, though he constructed very 
elaborate mouthpieces for his transmitters, lie paid 
but little attention to those of his receivers, though 
it seems obvious enough now that they were by far 
the most important, being directly concerned in the 
reproduction of the sounds. 

The first speaking telephone was invented by 
Professor Graham Bell, a naturalised American 
citizen, about four years after the appearance of 
Eeis's telephone. It consisted of two exactly similar 
instruments, one used as a transmitter and the other 
as a receiver, and its principle is easily understood 
by reference to Fig. 40. A steel 
magnet M is terminated at the end 
near the mouthpiece P by a piece 
of soft iron surrounded by a coil of 
very fine copper wire, covered with 
silk, and having its terminals per- 
manently connected to the binding 
screws S S, one of which is connected 
to the line wire and the other to 
earth. 

of soft iron D, tightly fastened at the P hone - 
edges, but with its centre free and nearly touching 

intermittent (when the current is periodically interrupted), pulsating 
(when its strength rises and falls suddenly), undulating (when its 
strength rises and falls gradually), and it is these last which are 
principally concerned in the transmission of human speech. 




Over the coil is a thin disc Yl %J?'~l*^ °l , ot 

Graham Bells Tele- 



270 ELECTRICAL APPLIANCES part iv 

the end of the magnet. Above the disc is the 
mouthpiece P spoken into in the case of the 
transmitter, and held to the ear in the case of 
the receiver. When the transmitter is used, the 
pressure of air against the thin iron diaphragm is 
altered with every inflection of the speaker's voice, 
and it is consequently set in vibration, making (to 
the eye,) imperceptible movements backwards and 
forwards in front of the wire coil, and thus starting 
alternate currents in it which are transmitted through 
the line wire to the coil of the receiving instrument, 
strengthening or weakening the magnetism of its 
core according to their direction. The result of this 
is to cause the second diaphragm to be attracted 
more or less strongly with every changing current, 
and so to vibrate in exactly the same manner as the 
diaphragm of the transmitter, imparting a similar 
motion to the air, and thus reproducing the sounds 
which originally started the currents. In fact, there 
is here a transformation of energy, precisely ana- 
logous to that which takes place between a dynamo 
and an electro-motor, only instead of mechanical 
motion being changed into electric currents by the 
dynamo and back again by the motor, we have 
sonorous vibrations transformed into electric currents 
in the telephonic transmitter, and restored to their 
original form in the receiver. The analogy can be 






chap. V THE TELEPHONE 271 

pursued further. Just as the same amount of work 
is not given out by the electro-motor that was ex- 
pended on the dynamo, so with the telephone also, 
loss occurs in the process of transmission, and the 
voice given out by Graham Bell's original apparatus 
was much weaker than that which had spoken in the 
first instance. For this reason the instrument was 
not altogether satisfactory, though it could and did 
transmit perfectly intelligible messages over a con- 
siderable distance. 

What was wanted in order to enable the telephone 
to attain the great practical importance which it has 
now reached, was the discovery of some means by 
which the weakening of the sounds transmitted 
could be obviated. Hughes, whose printing tele- 
graph was mentioned in the preceding chapter, accom- 
plished this by his invention of the microphone, 
some form of which is now invariably used as trans- 
mitter in telephonic circuits. The action of the 
microphone depends on the principle, that if the 
pressure between conductors in contact is altered, 
there is an alteration in the electrical resistance of 
the circuit of which they form a part ; so that if the 
pressure is lessened the resistance is increased and 
the current becomes weaker, while increase of pres- 
sure entails diminution of resistance and the current 
becomes stronger. Carbon is the substance best 



272 



ELECTRICAL APPLIANCES 



PART IV 



adapted to contacts of varying resistance, and 
carbon is consequently used in nearly all micro- 
phones, of which Fig. 41 represents a simple but 
very effective kind. 

When this instrument is placed in circuit with a 



ofO^nrnr^ 




TV 



Fig. 41. — Microphone. C, carbon pencil; B B, carbon blocks with slight 
depressions (not represented) on which the ends of the pencil loosely rest ; 
R, resonant board ; S, wooden slab ; iv w, connection to battery. 

telephone and used as a transmitter, the sounds 
emitted by the former are often very much louder than 
the original ones, simply because the varying pressure 
between the carbon pencil and its supports, so affects 
the currents as greatly to strengthen the vibrations 
to which they give rise in the receiving diaphragm. 
In fact, not only are audible sounds made louder, 
but sounds quite inaudible to the unaided ear are 
rendered perfectly distinct ; the walk of a fly even, 
over the soundboard of the microphone, being clearly 



chap. V THE TELEPHONE 273 

distinguished in the telephonic receiver, and words 
spoken at some yards distance from the former, being 
distinctly heard in the latter. 

The instruments most used in England are the 
" Blake " transmitter, a form of microphone, and the 
" Bell receiver, which is simply a Graham Bell tele- 
phone as described above. Besides a battery and the 
transmitting and receiving instruments, it is found 
advisable to include an induction coil in the circuit, 
the battery and primary coil being in connection 
with the transmitter, and the secondary coil with the 
receiver. 1 

Telephonic currents are calculated to be millions 
of times weaker than those used in ordinary tele- 
graphy, and their alternations are so excessively 
rapid, that there is no instrument delicate enough 
to make their presence known by optical *signs. 
Their ready detection by the ear is consequently a 
striking proof of its extreme sensitiveness to any 
periodic movement, however slight. 

The Telephone Exchange system, owing to which 
telephonic communication in our large towns has 
reached its present state of perfection, must not 
be passed over in silence. In principle it much 

1 In the original Graham Bell telephone no battery was needed, 
the currents being generated in the transmitter by the movements 
of the diaphragm in front of the magnet ; but where a micro- 
phone is used a separate generator is required. 



274 ELECTRICAL APPLIANCES part iv 



resembles an electric-lighting installation. Just as 
the latter has its central and sub-stations, so the 
Telephone Exchange has its central and local ex- 
changes connected by means of " trunk " wires. 
Each local exchange is connected to one or more 
others, and every subscriber has a private wire by 
means of which he can communicate with the local 
exchange to which he is attached. He is provided 
in his own house or office with a telephonic apparatus, 
consisting of a transmitter, a receiver, a battery, and 
a small magneto machine for making call signals, 
and a number by which he is to be known at the 
exchange is assigned to him. When he wishes to 
speak with another subscriber, he signals to his own 
local exchange, and if the second subscriber belongs 
to it also, they are put in immediate communication 
by having their respective telephonic circuits con- 
nected to each other. If the second subscriber 
belongs to a different local exchange, the signal is 
sent on to that, and communication is then opened 
between the persons who wish to converse by means 
of the two exchanges. If, however, the subscribers 
belong to local exchanges which are not connected 
with each other, the Central Exchange, with which 
all are connected, is signalled, and communication 
is set up through that. The institutor of this ex- 
ceedingly practical and well-devised system was the 



chap, v THE TELEPHONE 275 

father-in-law of Professor Graham Bell, Mr. Hubbard, 
who seems to have seen from the first, the great 
possibilities of his son-in-law's invention. 

The telephone is now often used for the trans- 
mission of music as well as of speech, whole concerts 
being made audible at distant places. Special 
arrangements are of course necessary in this case, as 
many considerations have to be attended to. It is 
necessary that the instrumental and vocal perform- 
ances should preserve their due proportion of sound 
w 7 ith regard to each other, that all extraneous sounds 
such as persons walking about the stage of the con- 
cert room, etc., should be excluded, and that the posi- 
tion of the singers should not interfere with the effect 
produced. All these difficulties have been success- 
fully surmounted, even that of sufficiently insulating 
the transmitting microphones from any sounds but 
the music. To effect this they are placed on thick 
layers of lead covered with gutta-percha. 1 

1 It is exceedingly difficult to insulate sound, and no substance 
so perfect for this purpose as gutta-percha is for insulating elec- 
tricity has yet been discovered. Professor Hughes, the inventor 
of the microphone, says, " The question of insulation has now 
become one of necessity, as the microphone has opened to us a 
world of sounds, of the existence of which we were unaware. If we 
can insulate the instrument so as to direct its powers on any single 
object, as on a moving fly, it will be possible to investigate that 
object undisturbed by the pandemonium of sounds which at present 
the microphone reveals where we thought complete silence pre- 
vailed." — Quoted from Electricity in the Service of Man, p. 723. 

19 



276 ELECTRICAL APPLIANCES part iv 

It is most common to employ overhead wires for 
telephonic purposes, but in some places (as in Paris) 
underground cables are used. In any case, it is most 
important if the telephone wires are near others 
employed for different electrical work — such as tele- 
graphy — to guard against the effects of induction, as 
the extreme sensitiveness of the telephone renders it 
specially liable to be interfered with by them, 1 and to 
have its true message interrupted or spoiled by sounds 
which have nothing whatever to do with it. A return 
wire (instead of the earth circuit), is generally found an 
efficient protection ; but other means are also used for 
underground wires, one of which is to sheath the tele- 
phone wire in an iron covering, so that induction shall 
take place in this, and leave the wire itself unaffected. 2 
" Earth currents " often produce disturbances in the 
telephone, causing a peculiar crackling noise, and 
thunderstorms give rise to very powerful effects. A 
flash of lightning too distant to be seen, may produce a 
sound in the telephone, and it is stated that this often 
occurs before the flash, showing that inductive action 
must have taken place previous to the discharge. 

1 It is, indeed, through using the telephone as a testing instru- 
ment that the fact before suspected has been proved, that any wire 
near another wire conveying an electric current has an induced 
current given rise to in it, however short the wires may be. 

2 It is equally necessary to guard telephone wires from induction 
by each other, or else messages sent by one travel along the others 
also, and the words are reproduced at several receivers. 



chap, v THE TELEPHONE 277 

The extreme sensitiveness of the telephone renders 
it very useful as a testing instrument for weak 
currents, there being apparently none so feeble that 
it will not give evidence of their presence ; and a 
special apparatus devised by Professor Hughes, and 
called the induction balance, has been made of great 
use for the instantaneous testing of metals. It con- 
sists of a battery connected to two small primary coils, 
near each of which is a secondary coil in circuit with 
a telephone. The arrangements are so made that 
the currents in the secondary coils are opposite, and 
exactly equal in strength. Now, two equal and 
opposite currents destroy each other, so while this 
condition of things obtains, there is no sound in the 
telephone, but if one or the other current is ever so 
slightly increased or diminished in strength, the 
equilibrium of the balance is disturbed and an electro- 
motive force arises, sufficient to cause a sound in the 
telephone. The testing of metals is thus carried 
out. Within each secondary coil is a box containing 
the specimens to be tested. If they are exactly 
similar, as for instance two genuine sovereigns 
would be, no sound is heard in the telephone ; but if 
a difference exists, such as that between a true and 
false coin, the currents in the secondary coils no 
longer balance each other, and the telephone emits 
its warning note. 



278 ELECTRICAL APPLIANCES part iv 

The microphone has been made very serviceable 
for medical purposes, and special instruments such 
as the " miophone " and the " sphyginophone " have 
been constructed for the examination of the muscles, 
pulse, veins, arteries, etc., in the human body; and 
there is no doubt that in time the use of such appar- 
atus will become widely extended. 

During the last two years public attention has 
been much drawn to an invention as surprising and 
ingenious as that of the telephone, viz. the phono- 
graph, and though this instrument is not electrical, it 
can hardly be passed over in silence, for it was de- 
vised and perfected by the great American inventor, 
to whom we owe so many marvels of applied electrical 
science ; and, moreover, in its latest form the phono- 
graph is driven by small electro-motors, deriving the 
necessary current from a battery of one or two cells. 
Electricity has therefore been utilised for it, and 
moreover, it is stated that Edison is endeavouring to 
combine the phonograph with the telephone in such 
a way, that even if no listener is at hand the latter 
can record its message to be heard when convenient. 
The work of the phonograph is to store spoken 
words or any desired sound, and to reproduce them 
when required. In order to effect this, it is neces- 
sary that the sonorous vibrations should make a per- 
manent impression on some suitable substance, and 



chap, v THE TELEPHONE 279 

that this impression should be able to give rise to 
exactly similar vibrations to those which produced it. 
Edison's first phonograph consisted of a brass 
cylinder, turned by a handle, and on which was cut 
a spiral groove. A piece of tinfoil was wrapped 
round the cylinder, and over it was fastened a metal 
diaphragm, having a metal point attached which 
rested on the tinfoil. Above the diaphragm was a 
mouthpiece. When the latter was spoken into, the 
diaphragm was caused to vibrate, and the cylinder 
being set in revolution at the same time, the metal 
point made a series of indentations, the depth of each 
varying with the strength of the vibration causing it ; 
and this, of course, with the various inflections of 
the voice. These indentations were the record of the 
spoken words, and when the latter had to be repro- 
duced a mouthpiece was held over a diaphragm 
exactly similar to the first, but placed with its 
attached point on the opposite side of the cylinder, 
which was revolved as nearly as possible at 
the same rate as when the record was being made. 
The second diaphragm was thus thrown into vibra- 
tion exactly corresponding to that of the first, and 
the spoken words were reproduced. The sound was, 
however, thin and metallic, the record on the tinfoil 
wore out after being used a few times, and the instru- 
ment would only give satisfactory results in the hands 



280 ELECTRICAL APPLIANCES part iv 

of an expert, and not always then. Though interest- 
ing and ingenious, it was therefore only a scientific 
toy. In Edison's perfected phonograph, first ex- 
hibited in England in 1888, the brass cylinder of the 
original instrument is replaced by one of solid wax, 
and the metal point by a cutting style, so that, 
instead of a series of indentations, a wavy line is 
made in the spiral groove. The metallic twang and 
unevenness of speech is thus done away with, and 
the true timbre, and all the inflections of the voice 
are reproduced ; moreover, the record on wax is far 
more durable than that on the tinfoil, for one of these 
waxen cylinders may be used more than a thousand 
times, and yet show no sign of deterioration. Im- 
pressions can be taken of them to any number 
desired, so that without speaking the record more 
than once, it can be indefinitely multiplied, and since 
the mechanism for revolving the cylinders, etc., is 
exactly the same in every instrument, they can be 
sent about by post to any person who possesses a 
phonograph, and be made to reproduce their recorded 
speech or music for his benefit. Usually it is 
necessary to insert acoustic tubes in the ears in 
order to hear the phonograph, but the necessity for 
this can be obviated by placing a- suitable funnel on 
the instrument, and its sounds are then made audible 
to a roomful of people at once. Distinctness is, 



chap, v THE TELEPHONE 281 

however, lost by this process. The only obstacle 
which seems to lie in the way of the adoption of the 
phonograph for everyday purposes, is the fact that 
the cylinders require careful adjustment when put 
into the instrument, and therefore a certain amount 
of skill is necessary. It is hardly probable, however, 
that so slight a reason will long keep the use of the 
phonograph in abeyance. The graphophone which, 
though improved from Edison's original phonograph, 
was not perfected by himself, requires less skill in 
the adjustment of the cylinders. On the other hand, 
its intonation is not so perfect. 



CHAPTEE VI 

ELECTRO-METALLURGY AND MISCELLANEOUS APPLIANCES 

OF ELECTRICITY 

Electro-plating — Dynamos used to generate the necessary currents 
— Quantity of current the important consideration — The 
"bath" — Anode must be made of the same metal which is to 
be deposited — Electro typing — Method of obtaining facsimiles 
of medals — Of wood-engravings — Reduction of metals from 
their ores — Fusion of metals with high melting-points — 
Welding — Medical appliances of electricity — Sewer purification 
— Firing of submarine mines — Electric bells — Alarums — 
Clocks — Conjectured possibility of transmitting vision. 

It has already been stated that an electric current is 
able to separate metals from their solutions, and that 
the liberated metallic atoms are always deposited at 
the negative electrode, or kathode. This property of 
the electric current has been made of great practical 
use in the now familiar process of electro-plating, 
under which comprehensive term is usually included 
the covering of any metal with a coating of another, 
whether the latter be precious or not. 



chap, vi ELECTRO-METALLURGY 283 

Formerly batteries were used to generate the 
required currents, but it is now found better and more 
convenient to employ continuous-current dynamos for 
this purpose. They are, however, of a different kind 
from those used for electric lighting and transmission 
of power, in both of which a current of high electro- 
motive force is required. This is not necessary in the 
case of the deposition of metals. Quantity of current 
is there the important consideration, and the dynamos 
are constructed accordingly. The method employed 
is as follows. The object to be plated is made the 
negative electrode, and immersed in an electrolytic 
cell, known for this purpose as a "bath" and filled 
with an acid solution containing a salt of the metal, 
say silver, to be deposited. The positive electrode is 
made in this case of a plate of pure silver, and 
immersed in the same solution. The current is then 
passed through the bath, the electrolytic liquid is 
decomposed, oxygen is liberated at the anode, and 
silver deposited on the kathode, i.e. on the object to be 
plated, and the process is continued until a coating of 
silver of sufficient thickness is obtained. If the 
plating is to be of gold or copper, exactly the same 
method is followed, only the solution is different, 
and in each case must contain a salt of the metal to 
be deposited. The object of making the anode of 
the same metal, is that by its gradually dissolving in 



284 ELECTRICAL APPLIANCES part iv 

the acid it may replace that which is being deposited 
on the kathode. Otherwise all the metal in the 
solution would be used up, and the process come to 
an end before the requisite thickness of coating had 
been obtained. 

Electrotyping is another most important branch of 
electro-metallurgy. In this industry electrolytic 
deposition is used to obtain facsimiles of medals, 
wood-engravings, ordinary printing type, and even 
daguerreotypes. To obtain the facsimile of a medal, 
a cast of the latter is first taken in some suitable 
substance, which if non-conducting h rubbed over 
with metallic powder to make it conduct. It is then 
immersed in a " bath," containing a solution of copper, 
and forms the negative electrode. The positive 
electrode consists of a solid bar or plate of copper. 
The current is then started and continued till a thick 
coating of copper is deposited on the mould, from 
which it can afterwards be easily detached. For 
facsimiles of wood-engraving, a mould of gutta-percha 
is first taken from the block itself, and then subjected 
to electrolytic deposition of copper for twenty-four 
hours. The engraving is then found to be reproduced 
on a very thin plate of copper, which is strengthened 
by having melted type-metal run in at the back. 
From the " electrotype " many thousand impressions 
can be taken, and it is chiefly owing to the facility 



chap, vi ELECTRO-METALLURGY 285 

and accuracy of the process, combined with its 
comparative inexpensiveness, that the number of 
illustrated books has so greatly increased of late years. 
When all impressions had to be taken from the block 
itself, but few good ones could be obtained, as the 
wood-engraving rapidly wore out. 

The intense heat of the electric arc causes it 
to be extensively used in the reduction of metals 
from their ores. Even the most refractory yield to 
this treatment, but the current required in order to 
produce the necessary amount of heat, often attains 
several thousand ampferes, the electro-motive force 
being, however, quite low. Currents of the same 
kind are also used for the fusion of metals with high 
melting-points, and for welding, complete success 
having been attained even with aluminium, though 
ail other means proved inadequate. For welding, 
however, it is not the electric arc which is used, but 
what may be called an "incandescent" method. 
The ends of the two pieces of metal to be welded are 
pressed together and the current passed through 
them, when the resistance it encounters at the point 
of contact, causes the development of a heat sufficient 
to soften the metals, so as to allow of their being 
easily united. 

The best known and most important practical 
appliances of electricity have now been touched on, 



286 ELECTRICAL APPLIANCES part iy 

but there are many minor uses (some of which should 
perhaps hardly be called minor,) which deserve at 
least mention. 

In the first place, attempts are increasingly made, 
and with increasing success, to use electric currents 
as curative agents, 1 and there can be little doubt that, 
as the natural electrical condition of the human body 
becomes better understood, they will prove of the 
utmost importance for medical purposes. Paralysis 
and kindred diseases are very frequently treated by 
electricity now ; but no such course should ever be 
pursued except by the advice, and under the 
direction of a properly qualified medical practitioner 
who has made a study of the subject. Amateur 
attempts, when not positively dangerous, are far 
more likely to result in harm than good to the 
patient for whose benefit they are intended. Besides 
paralysis, certain kinds of tumour and aneurism have 
been cured by means of electricity. For this purpose 
electrolysis is employed ; it sets up chemical processes 
in the affected parts which result in the dispersion of 
the tumour or the hardening of the aneurism. 

There is a hope that by means of a recent inven- 
tion, electrolysis may be utilised for a yet more 
important purpose than the cure of disease, viz. 
its prevention on a large scale through ridding sewers 

1 Induction currents are most frequently employed. 



chap, vi MISCELLANEOUS APPLIANCES 287 

of their poisonous gases, and thus rendering them 
innocuous. Electric currents have been also experi- 
mentally applied with some success to agriculture. 

From these beneficial uses, it seems sad to turn 
to the destructive purposes which electricity is made 
to serve in warfare. It is now almost the sole agent 
employed for firing submarine mines and torpedoes, 
and many most ingenious contrivances have been 
devised, enabling an operator on shore to explode a 
mine some distance out at sea, at the very moment an 
enemy's vessel is passing over it. 

Besides lighting, electricity is made to serve various 
domestic purposes. Electric bells are now so common 
and so evidently superior to all other kinds, that they 
will soon have no rivals left. Electric alarums are 
also frequently used in large establishments, and in 
some cases electrically-controlled clocks. There seems 
indeed no end to the ways in which man may employ 
this marvellous and ubiquitous agent, so mighty in 
its resources and yet so easily controlled. It has 
even been hoped that by its means vision, as well 
as speech, might be reproduced at a distance, and in 
fact some partially successful experiments in this 
direction, were made more than ten years ago by 
Professors Ayrton and Perry. The problems to 
solve are so many and so intricate, however, and the 
constructive difficulties to be overcome so great, that 



2SS ELECTRICAL APPLIANCES part iv 

it hardly seems possible an invention should ever be 
made which would be to the eye what the telephone 
is to the ear. Nevertheless, the marvellous adapta- 
tions of applied electrical science have astonished us 
so often, that it may well be another surprise is in 
store for us here. 



CONCLUDING CHAPTEE 

"WHAT IS ELECTRICITY ? " 

Impossibility of giving a categorical answer to the question — 
Analogy with the case of gravity — Our knowledge of electricity 
in reality greater — Possible connection between the two sets of 
phenomena — Important results achieved by electrical science 
— Connection between electricity and light — Electro-magnetic 
disturbances propagated through space with the velocity of 
light — Space not empty — The ether — Meaning of radiation — 
Of "radiant" heat, light, and electricity — Electro -magnetic 
theory of light — Undulatory theory of light — Experimental 
proofs on which it rests — Necessity for similar proof in the 
case of electro -magnetic undulations — Hertz's experiments — 
Their conclusive result and consequent immense importance 
to physical science — Concluding remarks. 

To the question " What is Electricity ? " it is im- 
possible as yet to give any certain answer — as im- 
possible as to say what the force of gravity is ; for 
though the fact is perhaps not generally realised, we 
know as little of the nature of the one agent as the 
other. We say that "electricity" or electrified 
bodies " attract " j we say that gravity is a force of 



290 ELECTRICAL APPLIANCES part 

attraction ; but neither of these assertions is an 
explanation, it is simply a statement of facts. In 
reality more is known about electricity than about 
gravity, for it is ascertained to some extent in what 
way the electrical forces are propagated, whereas 
w T ith respect to gravity, we are absolutely ignorant as 
to its manner of working. "We simply know that its 
observed effects are governed by unalterable laws, 
and it may perhaps be said, that the only hope of 
discovering anything further respecting its action, 
lies in the direction of there being found a connection 
between the phenomena of gravity and those of elec- 
tricity, a possibility which gives a yet additional 
interest both to the experimental and theoretical 
study of the latter. Already a great part of the 
domain of physics, and nearly all that of chemistry 
has been revolutionised by the growth of electrical 
science, and if somewhat extravagant expectations 
are entertained about its. future possibilities, they 
seem justified by the achievements it has already 
made. 

In the present work but little has been formally 
said respecting any theory as to the nature of 
electricity, but it has been intimated that the elec- 
trical forces are propagated through the ether, and 
that there is a very close connection between elec- 
tricity and light. To the observations already made, 



iv WHAT IS ELECTRICITY? 291 

it may be added that certain substances, notably 
selenium, have their electrical resistance very con- 
siderably altered if exposed to the action of light, also 
that a polarised ray of light in a strong magnetic field, 
has its angle of polarisation twisted into a different 
position. 1 But by far the most important point of 
connection between these two mysterious agents, 
light and electricity, is that the velocity with which 
an electro-magnetic disturbance is propagated through 
space, is equal to the velocity of light, being in round 
numbers, 180,000 miles a second. 2 Travelling at this 
rate light takes eight minutes to reach the earth from 
the sun, and the fact of its thus requiring time to 
travel, is sufficient to show us that space is not, as it 
is so often supposed to be, empty, but that there is 
something present in it which transmits light, and 
we may add, transmits also heat and electricity. The 
structure of this something, which it has been agreed 
to call " ether," is now, and perhaps always will be, 
unknown, but it is understood that it must be totally 
different to that of any other form of matter with 
which we are acquainted, though it appears to possess 
what may be called the counterparts of ordinary 
material qualities, such as elasticity and density. 

1 See note at the conclusion of the chapter, p. 300. 

- By an electro-magnetic disturbance is meant such a disturbance 
as is given rise to, for instance, by the discharge of a Leyden jar, 
or the make and break of a galvanic circuit. 

20 



292 ELECTRICAL APPLIANCES part 

Heat, light, and electrical energy are propagated 
through this medium in a way known as radiation, 
i.e. in spherical waves. The term " radiant heat " 
is becoming familiar even to the non-scientific, but 
it must not be supposed that it designates any par- 
ticular kind of heat. It simply means heat travelling 
in a particular kind of way, during which it does not 
appear as heat at all, but as motion ; and the same is 
true of light and of " radiant " electricity. There is no 
light,as we understand light, except where the presence 
of gross matter modifies the action of the ether ; the 
interplanetary and interstellar spaces are dark. 
Neither is there any sensible manifestation of heat or 
of electrical energy. But there is motion; and this 
motion is capable, under the right circumstances, of 
appearing either as light or heat or electrical energy. 
If, however, light and electrical undulations are 
propagated through the same medium with the same 
velocity in the same manner, it is not difficult to con- 
ceive that they must be more than merely related ; 
they must be to a great extent identical. This has 
been recognised by advanced scientific thinkers since 
the time — now more than twenty-five years ago — 
when the great English physicist and mathematician, 
Clerk Maxwell, published his Electro- Magnetic Theory 
of Light, of which the first experimental proof was 
given in 1877 by Professors Ayrton and Perry, who 



iv WHA T IS ELECTRICITY ? 293 

found that the ratio of the electro-magnetic to the 
electro-static unit of electric quantity 1 was equal to 
the velocity of light, as Clerk Maxwell had shown 
that it must be, if light were propagated as an 
electro-magnetic disturbance. In 1888 another great 
step was made, for the existence of electro-magnetic 
waves, in all respects similar to those of light, was 
detected by direct experiment, and that date conse- 
quently marks a memorable era in the history of 
modern science. In order that the reader may be 
enabled to comprehend the nature of these now cele- 
brated experiments, first carried out by a young 
German physicist named Hertz, and since him by 
others, reference must be made to what is known as 
the "undulatory theory " of light, and to the proofs 
on which it rests. 

For a very considerable time it has been under- 
stood that light is caused by motion of some kind. The 
question was whether that motion was one of material 
particles shot out from the luminous body, as Newton 
supposed, or whether it was a vibratory movement 
started by the luminous body in the ether, through 
which it was propagated. The latter theory was 
always found by far the most capable of explaining 
luminous phenomena, and experimental proof of it 
was finally given by two eminent scientific men, 
1 Generally known as "v." 



294 ELECTRICAL APPLIANCES part 

Young and Fresnel, the one English and the other 
French, independently. Connected with any wave 
movement, there is a phenomenon called inter- 
ference, which is caused by the action of waves 
on each other, and which can be roughly observed 
by any one watching water-waves turned back 
from a sea-wall upon those still advancing. It 
will be noticed that the direct and reflected waves 
meet each other in one of two ways, either crest 
joins crest and furrow furrow, with the result of 
making a much larger wave, or crest joins furrow 
and furrow crest, and there is a patch of comparatively 
calm water. A much better way of observing the 
effects of interference is to suspend a cord to the ceil- 
ing of a room, and taking the free end in the hand im- 
part to it a series of periodical impulses. AVaves are 
thus given rise to in the cord which run up to the 
ceiling, are there reflected, and return to the hand of 
the operator, and while this is going on, if the rate 
of motion of the hand be right for the length and 
mass of the cord, the latter will divide itself into a 
regular succession of loops, the centre of each one of 
which is a point of greatest amplitude of vibration, 
i.e. where the swing of the cord particles from side 
to side is widest, while between each loop is a point 
of no vibration, technically called a node. Where 
the loops occur, the direct and reflected waves meet 



iv WHAT IS ELECTRICITY? 295 

each other in what is called the same phase of vibra- 
tion (supposing they were water-waves, both would 
be rising and both falling at the same time) ; where 
the nodes occur, they meet in opposite phases of 
vibration (in water-waves one would be rising and 
one falling), so that the same point being acted on 
by two equal and contrary forces, remains at rest. 

Now, if light travels in waves, w 7 herever direct 
and reflected waves of light meet each other, there 
must be a similar phenomenon to that exhibited in 
the cord, and it can be observed by causing two 
pencils of red, or of any monochromous light to pass 
through two small apertures into a darkened room, 
and fall upon a white screen in such a way that 
certain portions of it are illuminated by reflected 
rays from both pencils, care being taken, as regards 
each pencil, that only light of one phase should reach 
the illuminated parts of the screen. There then appear 
on the latter alternate bands of the coloured light and 
of darkness, which correspond exactly to the loops and 
nodes in the cord, the coloured bands being caused 
by those light-waves which meet in the same phase 
of vibration, and the dark by those which meet in 
opposite phases. That this is really the case can be 
proved by shutting off the light from one of the 
pencils, when the dark bands disappear, thus showing 
that they were caused by interference. This experi- 



296 ELECTRICAL APPLIANCES part 

ment places beyond a doubt the truth of the undu- 
latory theory of light ; and in order to prove the 
electro-magnetic theory of light, or in other words, 
the existence of electro -magnetic undulations iden- 
tical with those of light, an experiment of the same 
kind, but of an electrical nature, was necessary. 

The difficulty lay in the extraordinary rapidity of 
vibration which it was necessary to attain, in order 
to procure waves manageable within the walls of 
a laboratory. The ordinary oscillations of a Leyden 
jar discharge, which are at the rate of one million a 
second, were far too slow, for the ether wave length 
corresponding to this period (viz. the millionth of a 
second,) is 300 metres, and one of a few feet only was 
required. To obtain this it was necessary to produce 
vibrations of the order of 100 million per second, 
and even when this difficulty had been surmounted, 
by the construction of a special apparatus whose 
capacity, resistance and self-induction were so pro- 
portioned as to give vibrations of the required 
rapidity, there remained the question, how could they 
be observed ? They could not be seen. Ether-waves 
which can be seen are waves of light, and their 
rate of vibration is several billion times a second. 
They could not be heard. To render them 
audible they must be made very much slower, as 
was done by Dr. Oliver Lodge in the experiment 



iv WHAT IS ELECTRICITY? 297 

described in Chapter iv. of Part III. In order to 
observe electrical waves too long to be seen, and too 
short to be heard, Hertz made use of the principle 
of resonance, the most familiar example of which is 
that of a tuning-fork, which when sounded is able 
to make any neighbouring tuning-fork of the 
same pitch as itself sound also, simply because their 
periods of vibration being the same, the second tun- 
ing-fork absorbs the vibrations of the first, and gives 
out exactly similar ones. Hertz therefore constructed 
two electrical circuits whose periods of vibration 
were identical. One he called the oscillator, and the 
other the resonator. The latter included a small 
spark gap, and was placed between the oscillator and 
the wall. When vibrations were set up in the oscil- 
lator, induced vibrations were set up in the resonator, 
and in a darkened room an infinitesimal spark was 
seen to cross the gap. By moving the resonator 
about to different parts of the room, Hertz found 
that in some places the sparking was more active, 
and in some it ceased altogether, and he also found 
that these spots of electric activity and repose suc- 
ceeded each other at regular intervals, thus showing 
that they were due to the interference of direct and 
reflected waves, and were in fact a phenomenon 
exactly similar to the nodes and loops in the cord, 
and the dark and light bands described above. By 



298 ELECTRICAL APPLIANCES part 

further developments of his experiment Hertz was 
able to prove that these electro-magnetic waves are 
transverse like light-waves, viz. that the vibrations 
take place across the line of propagation ; and more- 
over, that every effect which can be observed with 
the one kind of waves can be be observed with the 
other. Thus both alike can be reflected, refracted, 
and polarised. Nothing is therefore wanting for the 
complete establishment of the " electro - magnetic 
theory of light." Electrical, like luminous pheno- 
mena, are all referable to the action of the ether, and 
we may say either that electrical science includes the 
whole of optics, or that optics includes the whole of 
electrical science, whichever way we like to put it. 
This is the commencement of a very splendid 
generalisation. It is without doubt one of the 
most important advances in physical science which 
has ever been made, and it is with a feeling of intense 
expectation that the question arises in our minds, 
"What will be the next?" Scientific men seem trem- 
bling on the verge of some discovery which will come 
nearer to the solution of the problems surrounding 
the ultimate nature of matter, than even a few years 
ago was deemed possible. "Will this discovery be 
made in our own generation, or in the next, or per- 
haps not for many more to come ? We cannot tell. 
At any moment it seems possible, and yet it may be 



iv WHA T IS ELECTRICITY ? 299 

long deferred; but present knowledge would lead 
us to suppose that when it does come, it will be as 
amazing in its simplicity as in the depth and wide- 
ness of its scope. Already some of our leading 
physicists are beginning to formulate the query, — 
infinitely more far-reaching than that which heads 
this chapter, — whether all existing things are not 
modifications of that ether, but a short time since 
deemed hypothetical, and which is now proved to be 
the medium through which we receive the life-giving 
and life-sustaining energy emanating from the sun. 
Startling as such an idea may be at first sight, there 
is an inexpressible grandeur in the conception which 
it leads us to form of the unity of design pervading 
the whole of creation, and of that Infinite Mind 
which human reason, — its faint reflection, — is being 
more and more taught to realise as underlying and 
interpenetrating the " material universe." 



NOTE ON THE POLAEISATION AND MAG- 
NETISATION OF LIGHT. 

It was mentioned in the preceding chapter that the 
vibrations of light are of the transversal kind, %.e. 
that they take place across the line of propagation, 
and in a ray of ordinary light, they are executed in 
all directions across this line. If, however, such a 
ray be caused to pass through a thin slice of tour- 
maline, or a Nicol's prism, it emerges with all its 
vibrations reduced to one plane, and is in the con- 
dition known as that of plane polarised light, the 
tourmaline, or Nicol's prism through which it has 
passed being called the polariser. The plane of 
polarisation can be discovered by placing a second 
slice of tourmaline, or Nicol's prism, called the 
analyser, in the path of the polarised ray, for a 
position can be found for the analyser, in which it 
is opaque to the incident polarised ray, whose light 
is consequently quenched altogether. This position 
is always at right angles to that of the polariser, to 



NOTE 301 



the plane of polarisation of the ray, and to the 
position in which the analyser would itself polarise 
light. 

By the " magnetisation " of light discovered and 
named by Faraday, the following effect is meant. 
If a bar of a certain kind of " heavy glass " l be placed 
across the poles of a powerful electro-magnet, and a 
ray of polarised light caused to pass through it, and 
enter an analyser on the opposite side, it is found 
that when the current is passing through the coils of 
the electro-magnet, the position in which the analyser 
causes darkness to occur, is not the same as when 
there is no current. This shows that the plane of 
polarisation of the ray has been twisted round under 
the magnetic influence, and proves a direct action of 
magnetism on light. The direction in which the 
plane of polarisation is twisted, is for most dia- 
magnetic substances the same as that of the mag- 
netising current, but for many magnetic substances, 
contrary to it. Further proof of the rotatory effect 
produced by magnetism on light is given by the fact 
that a polarised ray reflected from a magnet or an 
electro-magnet also has its plane of polarisation twisted 
round. If the reflection takes place from a pole, the 

1 A tube containing bi-sulphide of carbon will have the same 
effect, which can also be produced in a minor degree by other 
substances, and to a very small extent by air. 



302 NOTE 



plane of polarisation is turned in a contrary direc- 
tion to that of the flow of the magnetising current, if 
from a point at the side of the magnet, in the same 
direction, provided the planes of incidence and polar- 
isation are parallel. If they are perpendicular to 
each other, the direction of rotation is only the same 
as that of the magnetising current if the angle of 
incidence exceeds 75°. For lesser angles the rota- 
tion is opposite to the direction of the current. 



INDEX 



Abingdon, thunderstorm at, 93 

Accumulator, 43, 232, 233, 246 

Aclinic line, 131 

Action, of points, 14 ; chemical, 
37 ; in galvanic battery 145 ; 
in voltameter, 159 ; and reac- 
tion of currents, 185 

Agonic line, 129 

Air, an insulator, 13 ; discharge 
through rarefied, 21 ; electrical 
condition of, 62 ; medium of 
transmission of sound, 79 

Alarums, 287 

Aluminium, welding of, 285 

Amber, 3 

Ampere, memoria technica of, 167, 
172 ; laws of, regarding inter- 
action of currents, 186 ; table 
of, 186, 187 ; the, 203 

Analogy between potential and 
level, 32 

Analyser, the, 300 

Anions, 157 

Anode, the, 157, 283 

Antimony, junction between, and 
bismuth, 153 

Applicances, practical, 206 ; mis- 
cellaneous, 286 

Arago, 179 

Arc, voltaic, 223 

Area, unit of, 30 

Armature, 209 ; cylindrical, 210 ; 
drum, 216 ; disc, ib. 



Atmosphere, electrical condition 
of, 61 

Atoms, combination of, 2 ; pro- 
cession of charged, 156, 157 ; 
deposition of metallic, 157, 282 

Attraction, 2 ; electrostatic, 4 ; 
magnetic, 117 ; laws of, between 
currents, 171 

Aureole, 192 

Aurora, Borealis, 67, 68; Australis, 
69 ; elevation of, 88 

Ayrton and Perry, electroscope of, 
35 ; clock -meter of, 240 ; tel- 
pherage introduced by, 247 ; 
experiments on electric trans- 
mission of vision by, 287 ; ex- 
perimental determination of "v" 
by, 293 

Balance, the induction, 277 
Bath, the, for electro -plating, 283 
Battery, of jars, 48 ; galvanic, 139, 
142 ; polarised, 145 ; principle 
common to every galvanic, 147 ; 
storage or secondary, 148 ; float- 
ing, 173 
Bell, electric, 242, 287 

receiver, the, 273 

Biot's experiment, 28 
Bismuth is diamagnetic, 
used in thermopile, 153 
Blake transmitter, the, 273 
Bunsen's cell, 146 



116; 



304 



INDEX 



C.G.S. system, the, 198 

Cells, 139, 140 ; in series and in 
parallel, 141 - 144 ; constant, 
145, 146 ; electrolytic, 157, 283 

Centimetre, 198 

Changes, chemical, caused by dis- 
charge, 56, 84 

Charge, induced, 11, 12 ; by con- 
duction, 12 ; seat of, 28 ; cause 
of, 30 ; residual, 46 

Chemical action. See Action 

Chimes, electric, 18 

Chinese and the magnet, 130 

Circuit, a, 53 ; closed voltaic, 174 ; 
sparks on making and breaking, 
191, 192 ; resistance of, 200 ; 
earth, 254 

Clarke's machine, 209 

Clouds, potential of, 62, 64 ; 
electrified throughout, 71 ; alti- 
tude of thunder-, 88 

Coils of galvanometers, 168 ; of 
electro -magnets, 180 seq.; resist- 
ance, 200 ; of field magnets in 
dynamos, 217 

Columbus, 130 

Commutator, the, 215, 256 

Compass, the mariner's, 130 ; the 
inclination, 131 ; standard, 137 

Compound -wound dynamos, 218 

Condenser, 43 ; air-, 46 

Conductors, 8 ; non-, 9 ; discharge 
of, 10 ; pointed, 14, 101 ; prime, 
17; lightning, 22, 101, 262; 
insulated, 24 ; potential of, 32 ; 
capacity of, 42; effect of charged, 
56 ; rotation of liquid, 177 

Connection, earth, 43, 103 

Contact between metals, 37 ; 
-breakers, 191 

Convection, 156 

Cooke's telegraph, 253 

Core of electro-magnets, the, 179, 
182, 183, 184 

Coulomb, law of, 5 ; the, 203 

Cuneus of Leyden, 44 

Currents, electric, direction of, 



139 ; cause of, 141, 142 ; 
energy of, 148, 150 ; thermo- 
electric, 153 ; chemical effects 
of, 155 ; strength of, 159, 200 ; 
physiological effects of, 161 ; 
galvanic, 163 ; magnetic effects 
of, 165 ; amperian, 175 ; in- 
duction, 182, 187, 195 ; higher 
order of, 196 ; measure of, 197 ; 
dynamo, 220 ; accidents from, 
235 ; telegraphic, 264 ; tele- 
phonic, 273 ; as curative agents, 
286 

Daniell's cell, 146-264 
Davy, Edward, 254 

Sir H., 157 ; voltaic arc, 223 



De la Kive, 173 

Declination of the needle, 126, 
135 ; map, 129 ; compass, 130 

Decomposition, chemical, by cur- 
rent, 140, 156 

Deflections of magnetic needle, 
165, 166, 167 

Density of electricity, 30 

Diamagnetism, 117 

Dielectrics, 42, 144 

Difference, potential, defined, 32 ; in 
inductors of influence machines, 
33 ; in batteries, 142 

Discharge, glow, 20 ; brush, 20, 
80; luminous, 21, 192, 194; 
of Leyden jar, 22, 45, 49 ; 
between clouds, 72 

Distance, the sparking, 50 

Disturbances, electro - magnetic, 
129 

Dogcart, electric, 247 

Drigg fulgurites, 85 

Drops, electrified, 64 

Duplexing, 205 

Dust figures, 55 

Dynamo machines, 210, 211 ; 
magneto and self-exciting, 212 ; 
alternating, 215 ; continuous- 
current, 215, 233 ; requisites 
for, 217 ; driving of, 245 



INDEX 



305 



Earth, connection, 103 ; a mag- 
net, 127 ; currents, 133, 276 

Edison lamp, 227 

Effects, luminous, 19, 22 ; me- 
chanical, 54 ; of evaporation, 

63 ; electrical in New York, 

64 ; of rupture on magnet, 123 ; 
of earth's magnetic force, 134 ; 
chemical, 155 ; physiological, 
161, 286 ; magnetic, 165 ; rota- 
tory, 176, 301 ; of induction 
on telephone, 276 

Electric light, arc, 223 seq. ; in- 
candescent, 227 seq. ; harmless, 
235 ; advantages of, 235, 236 ; 
rich in chemical rays, 236 

Electricity, static, 1 ; atmospheric, 
2, 20, 57, 70 ; kinds of, 5-8 ; 
bound, 11 ; distribution of, 32 ; 
thermo-, 37 ; current, 37, 83 ; 
cause of manifestation of, 38 ; 
source of atmospheric, 62 ; vari- 
ations of atmospheric, 63 ; cur- 
rent of defined, 139 ; for street 
traffic, 147 ; practical appliances 
of, 206 ; a light-producer, 222 

Electrics, 3, 9 

Electrode, 140, 156 

Electrolysis, 157, 283, 286 

Electrolytes, 157 

Electro- meters, 34 

Electro-metallurgy, 283, 285 

Electro-motors, 207, 218 

Electron, 3 

Electrophorus, 24, 33 

Electroplating, 282 

Electroscope, 34 

Electrotyping, 284 

Elements, magnetic, 131 ; of bat- 
tery, 139 

Energy, of lightning, 105 ; storage 
of electrical, 148 ; transmission 
of, through insulating medium, 
150 ; waste of, in heat, 195, 
209, 217 ; distribution of elec- 
trical, 233 ; measurement of, 
239 



Engineering, electrical, 219 

Equator, magnetic, 47 

Eruptions, volcanic, 89 

Ether, the, 47 ; medium of trans- 
mission of light, 79 ; of magnetic 
forces, 120 ; properties of, 291 ; 
function of, 298 

Evaporation, 63 

Experiments, 10, 28, 29, 44, 45, 
46, 52, 54, 58, 59, 60, 64, 83 
268 

Farad, the, 204 

Faraday, 29 ; on insulating med- 
ium, 31 ; discoverer of induc- 
tion currents, 187 ; on magnet- 
isation of light, 301 

Faults, telegraphic, 261 

Faure's storage battery, 147 

Fibres, bamboo, thread, 228 

Field, magnetic, 124-126, 172 
183, 184 

Filament, carbon, 228 

Fire, St. Elmo's, 20 

Flashes, lightning, cause of, 78, 
79 ; length of, 88 ; causes of, 
106 

Flashing, 229 

Fluctuations, magnetic, 133 

Fluorescence, 193 

Fog, electric, 62 

Force, magnetic, 117, 128 ; lines 
of, 125, 184 ; coercive, 135 
electro - motive, 143, 144 
thermo-electro-motive, 153,199 
measurement of, 198 

Fox, Lane (lamp), 228 

Franklin, theory of, 6 ; discovery 
of action of points, 14 ; on 
thunder and lightning, 57 

Fresnel, on light, 293 

Friction, 10 ; analogy with re- 
sistance, 55, 144 

Fulgurites, 85, 86 

Fuses, safety, 233 

Galvani, 139, 161 



306 



INDEX 



Galvanometers, 168, 169 ; mirror, 
170, 260 ; calibration of, 171 

Galvanoscope, 167 

Geissler's tubes, 21, 193 ; experi- 
ment on luminous discharge 
through, 194 

Gilbert, Dr., 3 ; his experiments, 
4 ; error, 9 ; earth a mag- 
net, 127 ; De magnete, 193 

Globules, Plante's luminous, 83, 
84 

Glow, electrical, 222 

Gramme, the, 198 ; ring, 213-215 

Graphophone, the, 281 

Gravity, 289, 290 

Grothuss' theory, 160 

Grove's battery, 146 

Guericke's, Otto von, machine, 
15, 24 

Hairpins, electric, 237 

Harris's, Sir W. S., system of 

protection from lightniug, 99, 

102 
Heat, effects of, 4 ; development 

of, 22 ; a source of electricity, 

37, 81 ; mode of travel of, 

155 ; radiant, 292 
Heart, the, and lightning shock, 

92, note 
Hertz, 293 ; his experiments, 297 
Hughes's printing telegraph, 258 
Humboldt, account of magnetic 

carriages, 130 

Incandescent lamp, 227-230 

Inclination of the needle, 131 

Illumination, indoor, 232 

Induced charges, 14, 38 

Induction, electrical, 11, 12 ; 
cause of shock, 23 ; magnetic, 
118; self-, 152, 190; of cur- 
rents by currents, 187, 188 ; 
by magnet, 189 ; of currents in 
dynamos 211 ; in underground 
and submarine cables, 262, 263 ; 
in telephone wires, 276 



Induction-coil/ 191; sparks from, 
192 

Inductors, 33 

Inertia, 151, 152 

Instruments, telegraphic, 254, 255 
seq. 

Insulation, 35 ; of electric-light- 
ing wires, 234 ; of telegraph 
wires, 26, 262 ; submarine 
cables, 263 

Insulators, 8, 9 

Intensity, magnetic, 128 

Interference and its effects, 294, 
295 

Inventions. See Guericke, etc. 

Ions, 157 

Iron, a magnetic substance, 116 ; 
magnetised and demagnetised, 
124 ; used for core of electro- 
magnets, 130 

Isogonic lines, 129 

Jablockoff candle, 226 

Jar, the Leyden, 40, 163 ; dis- 
covered, 43 ; method of charg- 
ing, 44, 45 ; oscillatory dis- 
charge of, 48, 49, 50, 296 ; when 
inclined to burst, 49 ; discharge 
of, 164 ; E. M. F. of, 204 

Jet, 3 

Joudpore powder-magazine, 101 

Judd, Prof., on electricity in vol- 
canic eruptions, 90 

Kathions, 157 

Kathode, 157, 282 

Kew, 131 ; magnetic needle in 

1851, 133 
Kite, Franklin's, 58 
Krakatoa, 89 

Laminated magnets, 121 
Lamps, arc, 225 ; illuminative 

power of, ib, ; incandescent, 

227 
Launches, electric, 248 
Leclanchc cell, 264 
Length, unit of, 37 



INDEX 



307 



Lenz's law, 195 

Lichtenburg's dust figures, 55 

Light, incandescent, 126 seq. ; 
the arc, 224 seq. ; connection 
with electricity, 290 ; polarised 
ray, 291 ; caused by motion, 
293 ; note on polarisation and 
magnetisation of, 300 

Lighthouses, 224 

Lighting, electric, 222, 234 ; in- 
stallations, 225 ; currents for, 230 

Lightning, 10, 57 ; cause of, 71, 
78 ; characteristics of, 72 ; rods, 
ib. ; duration of, 73 ; colours 
of, 74 ; shape of forked, ib. ; 
path of, 75, 77 ; sheet, 80 ; 
globular, 80-83 ; summer, 81 ; 
chemical changes by, 84 ; heat- 
ing effects of, 85 ; explosions 
through, 87 ; magnetising power 
of, 88 ; at Krakatoa, 89 ; dangers 
from, 91, 96 ; protection from, 
91, 99, 101, 102 seq. ; destruc- 
tion from, 93 seq. ; energy 
developed by, 105 

Lines, isogonic, 129 ; isoclinic, 
131 ; of force, 125 

Living body, passage of electricity 
through a, 10, 164 

Lodestone, the, 112 

Lodge, Dr. O., experiments of, 
41, 52 

Machines, electrical, 11 ; effects 
of, 14 ; frictional, 15, 16 ; 
cylinder, 16 ; influence, 24 ; 
Wimshurst's, 26 ; Holz's, 27 ; 
magneto- and dynamo-electric, 
206 ; Siemens's, 213 

Magnet, natural, 111 ; origin of 
name, ib. ; permanent, 112 ; 
artificial, ib. ; ways of making, 
120 ; strength of, 122 ; lifting- 
power of, ib. ; compensating, 126 ; 
controlling, 170 ; electro, 181 

Magnetic needle, floating, 134 ; 
deflection of the, 165-167 

21 



Magnetisation, of iron and steel, 

122, 135, 179 ; of light, 301 
Magnetism, 37, 111 ; two kinds 

of, 114 ; earth, 133 ; residual, 

179, 210 ; electro-, 180 
Magnetite, 112 
Magneto - electric machines, 207, 

203, 209 
Magnets, properties of, 115 seq. ; 

electro-, 178, 179 ; fixed or 

field, in dynamos, 211 
Map, declination, 129 ; magnetic, 

132 
Mass, unit of, 37 
Maxim lamp, 228 
Maxwell, Clerk, electro - magnetic 

theory of light, 292,293 
Measurement of quantities, 198 
Medium, insulating, 13, 30, 150 ; 

dielectric, 47, 150 
Mega-volts, 203 
Megohms, 203 
Mercury, 156, 199, 202 
Meridian, magnetic, 127, 136 
Metals, contact series of, 37, 39 ; 

resistance of, 199 ; fusion of, 285 
Meter, volt-, am-, clock-, 240 
Micro-ampere, 203 ; -farad, 204 
Microphone, the, 271 ; insulation 

of the, 275 ; for medical use, 

278 
Milli-ampere, 203 
Mines, firing, 287 
Miophone, 278 
Moisture, influence of, 4, 35 
Molecules, 3 
Morse, 255 ; code, 256 ; sounder, 

259 
Muscle, contraction of, 162 
Musical notes, transmission of, 

268, 275 

Navigation, aerial, 249 
Needle, magnetic, 55 ; dip of, 131 
Negative electricity, 6, 8, 17 
Nerve, the optic, effect of current 
on, 163 



3o8 



INDEX 



Nitric acid, formation of, 85 

Node, a, 294 

Non-conductors, 9, 10, 156, 158 

Non-electrics, 3 

Norman's inclination compass, 131 

OErsted's experiments, 166, 253 

Oscillations of Leyden jar dis- 
charge, 48, 52 ; of lightning, 
79 ; of magnetic needle, 128 

Oscillator, Hertz's, 297 

Ozone, 23 ; production of, 56, 59 ; 
active form of oxygen, 66 ; a 
chemical agent, 66 ; generated 
by lightning, 84 

Pair, thermo-electric, 153 ; astatic, 
169 

Paralysis, 286 

Perforation of cardboard, 54 

Phenomenon of recoil, 51 

Phonograph, the, 278-280 

Phosphorescence, 193 

Pixii's machine, 207, 208 

Plante, on the Aurora sound, 69 ; 
on globular lightning, 83 ; his 
storage battery, 149 

Platinum, 37 ; in Grove's battery, 
146, 227 

Play ford, storm at, 87 

Polarisation of battery, 140, 149 ; 
of light, 300 ; plane of, 301 

Pole, positive, 21, 140 ; negative, 
21, 140 ; magnetic, 128 

Positive elect ricity, 5, 6, 11, 15 ; 
absence of, 66 

Potassium, 157 

Potential, 31 ; difference of, de- 
fined, 32; high and low, 32, 
37 ; dependent on, 33 ; of the 
air, 61 ; of clouds, 62, 64 ; in 
galvanic battery, 63 

Power, definition of, 241 ; trans- 
mission of, ib. ; loss in trans- 
mission, 243 ; requisites for 
electric transmission of, 244, 
245 



Pressure, electric, 220 

Prism, Nicol's, 300 

Proof-plane, 29 

Protection from lightning, 106, 



109 



area of, 109 



Quadruplex telegraphy, 264 
Quantity of electricity, alteration 
in, 12, 13 ; per unit of area, 
30 ; measure of, 198-201 

Radiation, 292 

Reaumur, 44 

Recoil, 51 

Recorder, the syphon, 260 

Reis, telephone of, 267 

Relationship between magnetism 
and current electricity, 174 

Relay, polarised, 157, 158 

Repulse, the ship, 99 

Repulsion, 4 ; between magnets, 
115 ; between currents, 185 

Requisites for good dynamos, 217 

Resin, 55 

Resistance, to oscillations, 51, 52 ; 
of lightning rods, 105 ; defined, 
144 ; mechanical, 195 ; rede- 
fined, 199, 200; coils, 200, 
238 ; boards, 238 

Resonance, 79, 297 

Resonator, Hertz's, 297 

Retentivity, 135 

Ritter, accumulators, 149 

Rods, lightning, 103, 104 

Romas, experiments of, 58 

Ruhmkorffs coil, 191 

Rupture of magnet, 123 

Sacco, lightning at, 93 

Sand, vitrified, 85 

Saturation of magnets, 121, 183 

Search lights, 224, 225 

Seat of charge, 28 

Second, the, 198 

Selenium, 291 

Series -Avinding, 217 

Shell, magnetic, 122 



INDEX 



309 



Ships, struck by lightning, 99 ; 
loss of, 136 

Shunt-winding, 217 

Siemens, 210 ; armature, ib. ; 
self-exciting dynamo, 212 

Solenoid, 180-184 

Sound, rate of travel, 81 

Sources of electricity, 37 

Spark, electric, 19, 56, 222 ; from 
induction coils, 22 ; from 
Leyden jar, 45 ; duration of, 
48 ; effect of light of, on charged 
conductors, 78 

Speed of transmission of telegraphic 
signals, 258 

Sphygmophone, 278 

Steeple- Ash ton, ball of fire at, 95 

Steinheil, discoverer of earth-cir- 
cuit, 254 

Storms, magnetic, 129 ; period- 
icity of, 131 

Striae, 22 

Storage, work of, 148 ; batteries, 
148, 149 

Substances, magnetic, 116 ; dia- 
niagnetic, 117 

Suez Canal, lighting of, 237 

Saltan, H.M.S., struck, 87 

Sun-spots, 133 

Swan lamp, 227, 228 

System, the C.G.S., 198 



Tanjore, the, struck, 100 

Telegraph, the, 206, 250 seq. 

Telegraphy, duplex and quadru- 
plex, 264 

Telephone, the, 267 - 271 ; ex- 
change system, 273, 274 

Telpherage, 247 

Tetanus, 163 

Thales, 3 

Theory, molecular, of magnetisa- 
tion, 123, 124 ; electro-mag- 
netic, of light, 292, 296 ; un- 
dulatory, of light, 295 

Thermopile, 153 



Thompson, Sir W., mirror galvano- 
meter, 170, 260 

Thompson - Houston system of 
lighting, 230-232 

Thunder, 57 ; cause of, 60, 81 

Time, unit of, 37 

Torpedoes, 287 

Tourmaline, 300 

Traction, electric, 219 

Tramways, electric, 245 

Transformers, 221, 232 

Translation, 255 

Tubes, Geissler's, 21, 193 

Tuning-fork, 79, 297 

Turpentine, 156 

Undulatory theory and proof, 
293, 295 

Undulations, 293, 296 

Units, electrostatic, 36 ; electro- 
magnetic, 37 ; practical, 197- 
199 ; absolute, 198, 203, 204 ; 
of power, 241 

Uranium glass, 193 

Vacuum, a dielectric, 47 ; dia- 
magnetic, 120 

Velocity of light and electro-mag- 
netic disturbances, 291 

Vermilion, 55 

Vibrations in Leyden jur, 52 ; of 
tuning-fork, 79 

Vision reproduced, 287 

Volt, the, 202 

Volta, 37-39 ; his first battery, 
251 

Voltameter, 158, 159 

Water, dammed up, analogy with 
an electric charge, 31 ; between 
level of, and potential, 32 ; 
between flow of, and cuiTent, 
151 ; pure, a non-conductor, 158 

Watt, the, 241 

Waves, electro - magnetic, 293 ; 
transverse, 297 



3io 



INDEX 



Welding, 285 

Westbury Downs, storm at, 82 
" What is electricity ? " 289 
Wheatstone's, Sir C, experiment, 

54 ; self-exciting dynamo, 212 ; 

telegraph, 253 
Whirl, electrical, 19, 54 
Wilde's machine, 209 
Windmill, electric, 18 



Wires, overheated, 235 ; telegraph, 

261 
Woolridge, Capt., at Krakatoa, 

89 
Work, 32, 142 ; unit of, 241 

Young on light, 293 

Zinc, 37 ; sulphate of, 145. 



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n^HINGS NOT GENERALLY KNOWN; A 

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THROUGH MAGIC GLASSES and other Lectures. 
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CONTENTS. 



The Magician's Chamber by Moon- An Hour with the Sun. 

light. An Evening with the Stars. 

Magic Glasses and How to Use Them. Little Beings from a Miniature 
Fairy Rings and How They are Made. Ocean. 

The Life-History of Lichens and The Dartmoor Ponies. 

Mosses. The Magician's Dream of Ancient 
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ROUND AND ABOUT SOUTH AMERICA: 

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other Sketches and Studies of Travel. By Frank Vincent. 
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¥jE LIFE OF AN ARTIST. By Jules Breton. 
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HISTORY OF ENGLAND IN THE EIGHT- 
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IS TORY OF THE PEOPLE OF THE 

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D. APPLETON & CO.'S PUBLICATIONS. 
ASTRONOMY WITH AN OPERA-GLASS. 

-4J- A Popular Introduction to the Study of the Starry Heavens 
with the Simplest of Optical Instruments. By Garrett P. 
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This is a unique book, quite alone in the field that it occupies. The call for a 
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Some of the compliments that the book h.2s received are these: 

" A most interesting and even fascinating book." — Christian Union. . 

"The glimpses he allows to be seen of far-stretching vistas opening out on every 
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entific value." — Nature. 

"Mr. Garrett P. Serviss's book, 'Astronomy with an Opera-Glass,' offers us an 
admirable hand-book and guide in the cultivation of this noble aesthetic discipline (the 
study of the stars)." — New York Home Journal. 

" The book should belong to every family library." — Boston Flome Journal. 

" This book ought to make star-gazing popular." — New York Herald. 

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Democrat. 

" By its aid thousands of people who ha^e resigned themselves to the ignorance in 
which they were left at school, by our wretched system of teaching by the book only, 
will thank Mr. Serviss for the suggestions he has so well earned out." — New York 
Times. 

" For amateur use this book is easily the best treatise on astronomy yet published " 
— Chicago Herald. 

" 'Astronomy with an Opera-Glass' fills a long-felt want." — Albany Journal. 

" No intelligent reader of this book but will feel that if the author fails to set his 
public star-gazing the fault is not his, for his style is as winning, as graphic, and as 
clear as the delightful type in which it is printed." — Providence Journal. 

" Mr. Serviss neither talks over the heads of his readers nor ignores the sublime 
complexity and range of his themes, but unites simplicity with scholarship, scientific 
precision with life-long enthusiasm, and a genuine eloquence with rare touches of hu- 
mor. Considered as a product of the publishing industry, the book is elegance itself." 
■ — The Chautauquan. 



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